Two-stage laser system for aligners

ABSTRACT

The invention relates to a two-stage laser system well fit for semiconductor aligners, which is reduced in terms of spatial coherence while taking advantage of the high stability, high output efficiency and fine line width of the MOPO mode. The two-stage laser system for aligners comprises an oscillation-stage laser ( 50 ) and an amplification-stage laser ( 60 ). Oscillation laser light having divergence is used as the oscillation-stage laser ( 50 ), and the amplification-stage laser ( 60 ) comprises a Fabry-Perot etalon resonator made up of an input side mirror ( 1 ) and an output side mirror ( 2 ). The resonator is configured as a stable resonator.

ART FIELD

The present invention relates generally to a two-stage laser system foraligners, and more particularly to a two-stage laser system well fit forsemiconductor aligners, which is reduced in terms of spatial coherence.

BACKGROUND ART

So far, two-stage laser systems comprising an oscillation-stage laserand an amplification-stage laser adapted to amplify laser light emittedout of the oscillation-stage laser have been known so far in the art forthe purpose of obtaining high outputs. Two modes, MOPA (MasterOscillator Power Amplifier) and MOPO (Master Oscillator PowerOscillator) are known for double-chamber laser systems. The former is amode having no resonator in the amplification stage, and the latter is amode having an unstable resonator in the amplification stage. The MOPAmode and the MOPO mode have merits and demerits over each other.

MOPA

-   (a) Low spatial coherence (merit). That is, given the same share    quantity (pinhole-to-pinhole spacing) in the beam transverse    direction, the visibility of interference fringes is low. Notice    that the share quantity and visibility will be explained later.-   (b) Low energy stability (demerit). This is because output    fluctuations are sensitive to fluctuations of synchronous excitation    timing between the chambers.-   (c) Output efficiency is lower than that of the MOPO mode; laser    (seed) energy from the oscillation-stage laser must be more than    that of the MOPO mode (demerit).-   (d) Thick spectral line width (demerit). This is because the latter    half of a laser pulse from the oscillation-stage laser contains a    lot more roundtrips, and so the spectral line width is too narrow to    amplify the tail of that latter half.    MOPO-   (a) High spatial coherence (demerit). That is, given the same share    quantity (pinhole-to-pinhole spacing) in the beam transverse    direction, the visibility of interference fringes is high.-   (b) High energy stability (merit). This is because output    fluctuations are insensitive to fluctuations of synchronous    excitation timing between the chambers.-   (c) Output efficiency is higher than that of the MOPA mode; laser    (seed) energy from the oscillation-stage laser can be less than that    of the MOPA mode (merit).-   (d) Fine spectral line width (merit). This is because the latter    half of a laser pulse from the oscillation-stage laser contains a    lot more roundtrips, and so the spectral line width is narrow enough    to amplify the tail of that latter half.

As described above, the MOPO mode is more favorable than the MOPA modesaving (a) spatial coherence; in other words, it will be more suitableas a light source for semiconductor aligners such as excimer laser or F₂laser, if proper action is taken to reduce the spatial coherence.

However, the MOPO mode has now been found to have problems inconjunction with the use of an unstable resonator as mentioned above.The problems will now be discussed at great length.

In what follows, the “oscillation-stage laser” will be tantamount to the“line narrowing oscillation-stage laser”. A MOPA system, and a MOPOsystem is basically made up of at least one oscillation-stage laser andone amplification stage or amplification-stage laser. When there is noresonator in the amplification-stage laser, that amplification-stagelaser is herein called the amplification stage with no resonance oflight. A system having a resonator in the amplification stage is calleda MOPO system. When there is a resonator in the amplification stage, theamplification stage functions as an amplification-stage laser withresonance of light. Accordingly, when the amplification stage iscompared with the amplification-stage laser, higher efficiencyamplification is achievable with the amplification-stage laser than withthe amplification stage, given equal excitation energy.

So far, the amplification-stage laser of an excimer laser MOPO systemhas incorporated an unstable resonator using a concave mirror having aseed light-introduction hole in its center as an input side mirror and aconvex mirror as an output side mirror. Such a concave mirror/convexmirror combination of the unstable resonator constitutes a telephotooptical system having a geometrical magnification factor. Having anoptical magnification of about 20, the unstable resonator is used forthe purpose of efficiently obtaining high-output, high-coherence laserlight in the MOPO system. Notice that the unstable resonator has so farbeen used primarily as a light source for physicochemical researches.

A system having an unstable resonator in an amplification-stage laserhas been proposed as a light source for semiconductor aligners, as setforth in patent publication 1. Although this unstable resonator has anoptical magnification reduced down to about 10, the inventors'experimentation has suggested that the spatial coherence is not reduceddown to any sufficient level.

That is, the object of using the unstable resonator in a conventionalMOPO system is to provide efficient amplification of seed light. Aconcave mirror that forms a part of the unstable resonator is located inthe amplification-stage laser to inject the seed light all over theamplification-stage laser gain area, thereby providing efficientamplification of the seed light.

Patent Publication 1

Patent No. 2820103

Non-Patent Publication 1

“Basics and Applications of Lasers”, translated by Hitoshi Mochizuki andtwo others, pp. 30-33 (published from Maruzen Co., Ltd. on Jan. 20,1986)

Non-Patent Publication 2

Sov. J. Quantum Electron. 16(5), May 1986, pp. 707-709

One of the specifications of much importance in a laser system foraligners is in-plane low coherence (spatial coherence) in a laser lightprofile section. This spatial coherence capability (coherence) isevaluated by comparison of the coherence of a partial beam profile at agiven constant distance (share quantity) A in the beam profile. Thatdistance indicated by A is a value determined by element-to-elementspacing, etc. in a fly-eye lens used to eliminate brightness variationsin an illumination system in a semiconductor aligner such as a stepper.Then, the spatial coherence at two points in the share quantity A isevaluated by visibility defined by the following formula:Visibility=(maximum fringe intensity I _(max)−minimum fringe intensity I_(min))÷(maximum fringe intensity I _(max)+minimum fringe intensity I_(min))  (1)

Notice here that the “fringe intensity” means the intensity ofinterference fringes upon interference of light from two pointes. FIG.71 is indicative in schematic of interference fringes of light from twopoints at a given share quantity A and their maximum and minimum fringeintensities I_(max) and I_(min), and FIG. 72 is indicative in schematicof interference fringes of light from two points at a given sharequantity and their maximum and minimum fringe intensities I_(max) andI_(min), with an added laser portion. More specifically, FIG. 72 is aschematic representation of an optical arrangement for the evaluation ofspatial coherence of a laser light source by a Young's interferometer aswell as interference fringes of light from two points at a given sharequantity (=pinhole-to-pinhole distance) and their maximum and minimumfringe intensities I_(max) and I_(min). Generally, the spatial coherenceis determined depending on the size and intensity distribution of alight source, as viewed from the position of a pinhole that is a pointof measurement.

FIG. 73 is indicative of the results of measurement of the visibility ofa line narrowing laser and the results of measurement in the case ofusing the line narrowing laser as an oscillation-stage laser to provideamplification in an unstable resonator amplification-stage laser, asobtained in the inventors' experimentation. These results teach that itis required to satisfy the condition that the share quantity from asemiconductor aligner be equal to or greater than A and the visibilitybe equal to or less than Vt. Usually, the visibility of a single linenarrowing laser satisfies this condition. However, when an unstableresonator having a magnification factor of 5 was used in theamplification-stage laser in this experimentation, the share quantityproviding a visibility equal to or less than Vt increased up to B.B≈5×A; the share quantity providing the desired visibility equal to orless than Vt increases by the magnification factor of the unstableresonator. In other words, in the arrangement of FIG. 72 wherein thelaser portion is added to FIG. 71, the spatial coherence is evaluatedwhile a beam-expanding optical system comprising a combined concavemirror and convex mirror is located between the laser light source andthe pinhole. In this case, the size of the light source, as viewed fromthe pinhole, decreases by the beam magnification factor. Thus, the sharequantity providing the same visibility equal to or less than Vtincreases by the magnification factor.

In view of the fact that when the unstable resonator is used in theamplification-stage laser, the share quantity increases by a quantitycorresponding to the magnification of that unstable resonator, theinventors have made further experiments, using a MOPO system with astable resonator the optical magnification of which is set at 1 usingplane mirrors as both input- and output-side mirrors. As a result, ithas been found that the share quantity A equivalent to that obtainedwith a single oscillation-stage laser, i.e., that of seed light can beachieved with a MOPO system using that resonator (FIG. 73). That is, theinventors have now discovered that as the unstable resonator is used inthe amplification-stage laser of the MOPO system, it causes the sharequantity to increase by the optical magnification factor of the unstableresonator, and that if a stable resonator is used, this can then beaverted. As described later, this finding is one of the rudiments of theinvention.

From another angle of view, why the spatial coherence and the sharequantity increase with the use of the unstable resonator is nowexplained.

FIG. 74 is illustrative in (beam profile) section of laser light emittedout of an oscillation-stage laser. Consider now the coherence of laserlight P1 and P2′ spaced away by a distance A1 and laser light P1 and P2spaced away by a distance A2 in the beam profile. As shown in FIG. 75,the laser light P1 and P2′ at a short distance are put in order orsubstantially equal in terms of wave phase. With increasing distance,however, there is a little wave phase shift even at the same wavelength;laser light P1 and P2 at a relatively long distance are less likely tointerfere spatially. In other words, a long pinhole-to-pinhole distanceallows for a decrease in the visibility of interference fringes.

In the prior art, the amplification-stage laser resonator was anunstable resonator. As shown in FIG. 78(a), the unstable resonatorcomprises an input side concave mirror and an output side convex mirror,and is of the type that is capable of geometrically expanding thesection of seed light. Accordingly, when both the amplification-stagelaser and the oscillation-stage laser are of much the same size inexcitation section (discharge section), seed light from theoscillation-stage laser is such that a partial beam portion having aradius 3A is cut out of the general beam section, as shown in FIG. 76.In the section cut out in this way, the closer the laser light P2 is tothe laser light P3, the higher the visibility of interference fringebecomes. Although there is a low visibility at a distance A3, thevisibility of interference fringe at distance A4 becomes higher withincreasing coherence.

As described above, the prior art amplification-stage laser resonator isan expander system; laser light is expanded while high coherence ismaintained. As a consequence, the post-amplification laser light P3diverges to the position of P3′ as shown in FIG. 77, while highcoherence is maintained intact. Thus, even when the specifications forcoherence are met at a distance A5 in the oscillation-stage laser, highcoherence is still maintained even at a distance A6 beyond the distanceA5 by the expansion of the beam of seed light after amplification,offering a problem that the specifications for low coherence are notmet.

FIG. 78 is illustrative of how seed light (explained with reference toFIGS. 76 and 77) diverges in the amplification-stage laser using anunstable resonator. A laser light section at a position Z1 of the inputside concave mirror (FIG. 78(b)) corresponds to FIG. 76, and a laserlight section at a position Z2 of the output side convex mirror (FIG.78(c)) corresponds to FIG. 77. In the prior art, the amplification-stagelaser resonator was an unstable resonator. As shown in FIG. 81(a), theunstable resonator comprises an input side cylindrical concave mirrorand an output side cylindrical convex mirror. Reference is now made to aresonator of the type that is capable of geometrically expanding thesection of seed light in a longitudinal direction. Seed light oscillatedout of the oscillation-stage laser is such that, as shown in FIG. 79,the visibility at a distance A3 (share quantity) between laser light P1and P3 is higher in the section of injection of seed light than in thesection of a beam in the amplification-stage laser amplified by theunstable resonator shown in FIG. 80. Given visibility equivalent to thatin the oscillation-stage laser, the distance A4 between laser light P1and P3 becomes long by the magnification factor of the unstableresonator, meaning that the spatial coherence becomes high.

With the prior art two-stage laser system for aligners that relies uponthe MOPO mode, the spatial coherence distance becomes long in proportionto the magnification factor at which the beam of seed light is expandedby the unstable resonator, because the unstable resonator is used in theamplification-stage laser. Thus, the prior art two-stage laser is lessthan satisfactory for light sources for semiconductor aligners.

DISCLOSURE OF THE INVENTION

In view of such problems with the prior art as described above, theprimary object of the invention is to provide a two-stage laser systemwell fit for semiconductor aligners, which is reduced in terms ofspatial coherence while taking advantage of the high stability, highoutput efficiency and fine line width of the MOPO mode.

According to the invention, this object is accomplished by the provisionof a two-stage laser system for aligners, comprising anoscillation-stage laser and an amplification-stage laser, wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

said oscillation-stage laser and said amplification-stage laser eachcomprises a chamber filled with a laser gas,

said oscillation-stage laser oscillates laser light having divergence,and

said amplification-stage laser comprises a Fabry-Perot etalon typeresonator, wherein said resonator is configured as a stable resonator.

Preferably in this case, the resonator comprises an input side mirror inwhich laser light oscillated out of said oscillation-stage laser isentered and an output side mirror through which amplified laser lightoutputs, wherein the input side mirror comprises a total-reflectionmirror having a total-reflection mirror coating externally of an areathrough which laser light oscillated out of the oscillation-stage laseris introduced in the resonator, and the output side mirror comprises aplanar, partial reflecting mirror.

The substrate of the input side mirror could be provided in itssubstantially central portion with a hole or slit shaped in such a wayas to introduce laser light oscillated out of the oscillation-stagelaser in the resonator.

The substrate of the input side mirror is formed of a transparentsubstrate, and a total-reflection mirror coating is applied to aperipheral area of the surface of the transparent substrate other thanan area at a substantially central portion of the surface of thetransparent substrate, wherein said area is shaped in such a way as tointroduce laser light oscillated out of the oscillation-stage laser inthe resonator, or a slit area including said shape.

The laser light oscillated out of the oscillation-stage laser could beintroduced in the resonator from a periphery of the input side mirror ora peripheral portion thereof that is not applied with a total-reflectionmirror coating.

Alternatively, the resonator could comprise an input side mirror inwhich the laser light oscillated out of the oscillation-stage laser isentered and an output side mirror through which the amplified laserlight outputs, wherein the input side mirror comprises a partialreflecting mirror and the output side mirror comprises a planar, partialreflecting mirror.

The laser light oscillated out of the oscillation-stage laser could beintroduced in the resonator from a periphery of the input side mirror.

The output side mirror in the resonator in the oscillation-stage laserand the input side mirror in the amplification-stage laser could beformed on each side surface of the same substrate.

The input side mirror comprises a plane mirror, a concave mirror or acylindrical concave mirror.

Alternatively, the resonator could comprise an output side mirror inwhich the laser light oscillated out of the oscillation-stage laser isentered and through which the amplified laser light outputs, and a rearside mirror, wherein the substrate of the output side mirror is formedof a transparent substrate, an area of the output side mirror, throughwhich the amplified laser light outputs, has a partial reflectioncapability, and the rear side mirror comprises a planar total-reflectionmirror.

The resonator could comprise an output side mirror in which the laserlight oscillated out of the oscillation-stage laser is entered andthrough which the amplified laser light outputs, and a right-angleprism, wherein the substrate of the output side mirror is formed of atransparent substrate, an area of the output side mirror, through whichthe amplified laser light outputs, has a partial reflection capability,and the right-angle prism comprises a total-reflection right-angle prismcapable of reflecting all incident light.

The laser light oscillated out of the oscillation-stage laser isintroduced in the resonator from a periphery of the output side mirroror a peripheral portion of the output side mirror having no partialreflection capability.

The output side mirror could be a partial reflecting mirror.

In this case, the laser light oscillated out of the oscillation-stagelaser could be introduced in the resonator from a periphery of theoutput side mirror.

The output side mirror could comprise a plane mirror, a concave mirroror a cylindrical concave mirror.

In the two-stage laser system for aligners according to the invention,the resonator could comprise an input side mirror in which the laserlight oscillated out of the oscillation-stage laser is entered, whereinthe input side mirror comprises a partial reflecting mirror, and anoutput side mirror, wherein the output light of the oscillation-stagelaser is entered in the resonator through the partial reflecting mirror,and the optical axis of the resonator is in substantial alignment withthe optical axis of the oscillation-stage laser.

The resonator could comprise a total-reflection rear side mirror and anoutput side mirror, wherein a beam splitter is located between the rearside mirror and a rear side laser window and on the optical axis of saidresonator, the laser light oscillated out of the oscillation-stage laseris incident on the beam splitter, and the optical axis of laser lightreflected from the beam splitter is in substantial alignment with theoptical axis of the resonator.

The resonator could comprise a total-reflection rear side mirror and anoutput side mirror, wherein a beam splitter is located between the rearside mirror and a front side laser window and on the optical axis of theresonator, the laser light oscillated out of the oscillation-stage laseris incident on the beam splitter, and the optical axis of laser lightreflected from the beam splitter is in substantial alignment with theoptical axis of the resonator.

The resonator could comprise a total-reflection rear side mirror and anoutput side mirror, wherein a beam splitter, on which the laser lightoscillated out of the oscillation-stage laser is incident, is locatedexternally of the resonator and on the optical axis of the resonator,the laser light oscillated out of the oscillation-stage laser isincident on the beam splitter, the optical axis of laser light reflectedfrom the beam splitter is in substantial alignment with the optical axisof said resonator, and the laser light is entered in the resonatorthrough the output side mirror.

A front mirror in the oscillation-stage laser could comprise a partialreflecting mirror, and be shared by an input side mirror in which thelaser light oscillated out of the oscillation-stage laser is entered.

In the two-stage laser system for aligners according to the invention,the optical axis of laser light oscillated out of the oscillation-stagelaser and entered in the amplification-stage laser could be set at anangle with respect to the optical axis of the resonator in theamplification-stage laser.

A length about twice as long as the length of the resonator in theamplification-stage laser could be set longer than a time-based coherentlength corresponding to the spectral line width of the oscillation-stagelaser.

The two-stage laser system for aligners according could further comprisebetween the oscillation-stage laser and the amplification-stage laser aconversion optical system having at least one of a function ofcompressing the beam shape of laser light oscillated out of theoscillation-stage laser and a function of magnifying the divergence oflaser light oscillated out of the oscillation-stage laser.

Preferably, the divergence of laser light entered in theamplification-stage laser should satisfy the following conditions:$\begin{matrix}\begin{matrix}{{\theta\quad h} \geqq {{Tan}^{- 1}\left\lbrack {\left\{ {\left( {{Ha} - {Hs}} \right)/2} \right\} \cdot {\left( {1/L} \right)/\left( {P \cdot {c/L}} \right\rbrack}} \right.}} \\{= {{Tan}^{- 1}\left\{ {\left( {{Ha} - {Hs}} \right)/\left( {2 \cdot P \cdot c} \right)} \right\}}}\end{matrix} & (2) \\\begin{matrix}{{\theta\quad v} \geqq {{Tan}^{- 1}\left\lbrack {\left\{ {\left( {{Va} - {Vs}} \right)/2} \right\} \cdot {\left( {1/L} \right)/\left( {P \cdot {c/L}} \right\rbrack}} \right.}} \\{= {{Tan}^{- 1}\left\{ {\left( {{Va} - {Vs}} \right)/\left( {2 \cdot P \cdot c} \right)} \right\}}}\end{matrix} & (3)\end{matrix}$Here θv and θh are the angles of divergence of laser light entered inthe amplification-stage laser in the vertical and horizontal directions,respectively, P is an effective pulse width, c is the velocity of light,L is a resonator length, Vs and Hs are the beam diameters of laser lightentered in the amplification-stage laser in the vertical and horizontaldirections, respectively, and Va and Ha are the beam diameters of outputlight in the vertical and horizontal directions, respectively.

The invention also provides a two-stage laser system for aligners,comprising an oscillation-stage laser and an amplification-stage laser,wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

the oscillation-stage laser and the amplification-stage laser eachcomprises a chamber filled with a laser gas,

the oscillation-stage laser oscillates laser light having divergence,

the amplification-stage laser comprises a ring resonator comprising aninput/output partial reflecting mirror and a plurality oftotal-reflection mirrors for reflecting laser light entered via thepartial reflecting mirror back to a position of the partial reflectingmirror, and

the partial reflecting mirror and the plurality of total-reflectionmirrors are each formed of a plane.

In this case, between the oscillation-stage laser and theamplification-stage laser there could be located a conversion opticalsystem having a function of compressing the beam shape of laser lightoscillated out of the oscillation-stage laser.

The optical path length in the ring resonator could be set longer than atime-based coherent length corresponding to the spectral line width ofthe oscillation-stage laser.

Further, the invention provides a two-stage laser system for aligners,comprising an oscillation-stage laser and an amplification-stage laser,wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

the oscillation-stage laser and the amplification-stage laser eachcomprises a chamber filled with a laser gas,

said amplification-stage laser comprises a Fabry-Perot etalon resonator,wherein the resonator is configured as a stable resonator, and

the optical axis of laser light oscillated out of the oscillation-stagelaser and entered in the amplification-stage laser is set at an anglewith respect to the optical axis of the resonator in theamplification-stage laser.

Further, the invention provides a two-stage laser system for aligners,comprising an oscillation-stage laser and an amplification-stage laser,wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

the amplified laser light leaves as output, wherein theoscillation-stage laser and the amplification-stage laser each comprisesa chamber filled with a laser gas,

the amplification-stage laser comprises a Fabry-Perot etalon resonator,wherein the resonator is configured as a stable resonator, and

a length about twice as long as the length of the resonator in theamplification-stage laser is set longer than a time-based coherentlength corresponding to the spectral line width of the oscillation-stagelaser.

Further, the invention provides a two-stage laser system for aligners,comprising an oscillation-stage laser and an amplification-stage laser,wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

the oscillation-stage laser and the amplification-stage laser eachcomprises a chamber filled with a laser gas,

the amplification-stage laser comprises a Fabry-Perot etalon resonator,wherein the resonator is configured as a stable resonator,

the optical axis of laser light oscillated out of the oscillation-stagelaser and entered in the amplification-stage laser is set at an anglewith respect to the optical axis of the resonator in theamplification-stage laser, and

a length about twice as long as the length of the resonator in theamplification-stage laser is set longer than a time-based coherentlength corresponding to the spectral line width of the oscillation-stagelaser.

Further, the invention provides a two-stage laser system for aligners,comprising an oscillation-stage laser and an amplification-stage laser,wherein:

laser light output from said oscillation-stage laser is injected intosaid amplification-stage laser and is amplified therein,

said amplified laser light is output from said amplification-stagelaser,

the oscillation-stage laser and the amplification-stage laser eachcomprises a chamber filled with a laser gas,

the amplification-stage laser comprises a resonator comprising a rearside mirror and an output side mirror,

the reflecting surfaces of the rear side mirror and the output sidemirror are each formed of a plane,

the normal lines to the rear side mirror and the output side mirror areset at an angle with respect to the optical axis of laser lightoscillated out of the oscillation-stage laser and entered in theamplification-stage laser and at an angle with one another, and

laser light oscillated out of the oscillation-stage laser is entered inthe resonator from a side on which a distance between both mirrors islonger.

Preferably in this case, the resonator should be positioned such thatlaser light reflected at the rear side mirror or the output side mirroron which laser light oscillated out of the oscillation-stage laser isfirst incident is reflected toward a side on which the distance betweenboth mirrors is shorter.

Preferably, the rear side mirror and the output side mirror should bemutually set in such a way as to make an angle of 0.01 mrad to 0.2 mradinclusive.

In this case, too, a length about twice as long as the length of theresonator in the amplification-stage laser should be set longer than atime-based coherent length corresponding to the spectral line width ofthe oscillation-stage laser.

Throughout the above two-stage laser systems for aligners, the laserlight oscillated out of the oscillation-stage laser could be introducedin the resonator from any side position of the resonator.

Throughout the above two-stage laser systems for aligners, each of themirrors that form the resonator could be held by a mirror holder capableof moving each mirror in a substantially vertical direction to theoptical axis direction of the resonator.

Throughout the above two-stage laser systems for aligners, theoscillation-stage laser could further comprise line narrowing means forline narrowing the oscillated laser light so as to be configured as aKrF excimer laser, an ArF excimer laser, and a molecule fluorine (F₂)laser.

Alternatively, the laser system could be configured as a moleculefluorine (F₂) laser system comprising wavelength select means forselecting one oscillation line from laser light oscillated in theoscillation-stage laser.

Still alternatively, the laser system could be configured as a moleculefluorine (F₂) laser system comprising wavelength select means forselecting one oscillation line from laser light produced on the outputside of the amplification-stage laser.

In the two-stage laser system for aligners according to the invention,oscillation laser light having divergence is used as theoscillation-stage laser and the amplification-stage laser comprises aFabry-Perot etalon resonator where the resonator is configured as astable resonator or, alternatively, oscillation laser light havingdivergence is used as the oscillation-stage laser and theamplification-stage laser comprises a ring resonator comprising aninput/output partial reflecting mirror and a plurality oftotal-reflection mirrors for reflecting laser light entered via thepartial reflecting mirror back to the position of the partial reflectingmirror wherein the partial reflecting mirror and the plurality oftotal-reflection mirrors are each formed of a plane. Thus, the two-stagelaser system for aligners according to the invention has the features ofthe MOPO mode that output fluctuations are insensitive to fluctuationsof synchronous excitation timing between the chambers, high energystability and high output efficiency are achievable, laser (seed) energyfrom the oscillation stage can be kept lower, the spectral line width isnarrow because of the latter half of a laser pulse from theoscillation-stage laser makes a lot more roundtrips, and the line widthis narrow because the tail of the latter half can be amplified, and hasthe features of the MOPA mode as well that the spatial coherence is low;that is, given the same share quantity (pinhole-to-pinhole space) in thebeam transverse direction, the visibility of interference fringes andthe spatial coherence are low.

If the optical axis of laser light oscillated out of theoscillation-stage laser and entered in the amplification-stage laser isset in such a way as to make an angle with respect to the optical axisof the resonator in the amplification-stage laser, then the spatialcoherence is much more reduced.

If the length about twice as long as the length of the resonator in theamplification-stage laser is set longer than the time-based coherentlength corresponding to the spectral line width of the oscillation-stagelaser or the length of the optical path through the ring resonator isset longer than the time-based coherent length corresponding to thespectral line width of the oscillation-stage laser, it is then possibleto prevent any interference fringe pattern from occurring on the beamprofile of laser light produced out of the amplification-stage laser. Itis thus possible to maintain the symmetry of the beam profile and holdback its fluctuations and, hence, provide uniform illumination of masksin an aligner. Thus, the invention provides a two-stage laser systemwell fit especially for semiconductor aligners.

The invention is in no sense limited to the use of the oscillation laserlight having divergence as the oscillation-stage laser. For instance, ifthe optical axis of laser light oscillated out of the oscillation-stagelaser and entered in the amplification-stage laser is set in such a wayas to make an angle with respect to the optical axis of the resonator inthe amplification-stage laser, it is then possible to obtain a two-stagelaser system that does not only have the above features of the MOPO modebut also is reduced in terms of spatial coherence so that it lendsitself well to semiconductor aligners.

Further, if the reflecting surfaces of the rear side mirror and theoutput side mirror are each formed of a plane, the normal lines to therear side mirror and the output side mirror are set in such a way as tomake an angle with respect to the optical axis of laser light oscillatedout of the oscillation-stage laser and entered in theamplification-stage laser and with each other as well, and the laserlight oscillated out of the oscillation-stage laser is entered in theresonator from the side on which the distance between both mirrors islonger, it is then possible to obtain a two-stage laser system that doesnot only have the above features obtained by setting the optical axis oflaser light entered in the amplification-stage laser in such a way as tomake an angle with respect to the optical axis of the resonator in theamplification-stage laser but also has an increased laser output and anextended pulse width and ensures the degree of flexibility in theinjection of laser light entered in the amplification-stage laser with adecrease in the peak intensity of the oscillation-stage laser, and so isbest suited for use with semiconductor aligners.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustrative in schematic of the fundamental construction ofthe two-stage laser system for aligners according to the invention.

FIG. 2 is illustrative of how the divergence needed for seed light inthe invention is defined.

FIG. 3 is illustrative of the construction of one embodiment of theresonator mirrors used in the amplification-stage laser.

FIG. 4 is illustrative of the construction of another embodiment of theresonator mirrors used in the amplification-stage laser.

FIG. 5 is illustrative of the construction of yet another embodiment ofthe resonator mirrors used in the amplification-stage laser.

FIG. 6 is illustrative of the construction of a further embodiment ofthe resonator mirrors used in the amplification-stage laser.

FIG. 7 is illustrative of the construction of a further embodiment ofthe resonator mirrors used in the amplification-stage laser.

FIG. 8 is illustrative of the construction of a further embodiment ofthe resonator mirrors used in the amplification-stage laser.

FIG. 9 is illustrative of the conversion optical system interposedbetween the oscillation-stage laser and the amplification-stage laser.

FIG. 10 is illustrative of one exemplary conversion optical system.

FIG. 11 is illustrative of another exemplary conversion optical system.

FIG. 12 is illustrative of the two-stage laser system for alignersaccording to the invention using another conversion optical system.

FIG. 13 is a general representation of one embodiment of the two-stagelaser system for aligners comprising the basic features of theinvention.

FIG. 14 is illustrative of part of one specific embodiment of thetwo-stage laser system for aligners of FIG. 13.

FIG. 15 is illustrative of share quantity of low coherence versusvisibility relations in the arrangement of FIG. 14.

FIG. 16 is illustrative of the two-stage laser system for aligners,which comprises one modification to the input side mirror in theamplification-stage laser.

FIG. 17 is illustrative of another modification to the input side mirrorin the resonator used in the amplification-stage laser.

FIG. 18 is illustrative of yet another modification to the input sidemirror in the resonator used in the amplification-stage laser.

FIG. 19 is illustrative of the two-stage laser system for aligners,which comprises another modification to the input side mirror in theamplification-stage laser.

FIG. 20 is illustrative of the principles of operation when the opticalaxis of the resonator in the amplification-stage laser and the opticalaxis of seed light make an angle.

FIG. 21 is illustrative of the two-stage laser system for aligners,which comprises yet another modification to the input side mirror in theamplification-stage laser.

FIG. 22 is illustrative of a modification to the input side mirrorusable in the embodiment of FIG. 19.

FIG. 23 is illustrative of a modification to the input side mirrorusable in the embodiment of FIG. 20.

FIG. 24 is illustrative of a modification to the input side mirrorusable in the embodiment of FIG. 21.

FIG. 25 is illustrative of part of another embodiment of the two-stagelaser system for aligners according to the invention.

FIG. 26 is illustrative of the amplification-stage laser that forms apart of the two-stage laser system for aligners shown in FIG. 25.

FIG. 27 is illustrative of one exemplary mirror holder.

FIG. 28 is illustrative of part of yet another embodiment of thetwo-stage laser system for aligners according to the invention.

FIG. 29 is illustrative of another embodiment of the amplification-stagelaser in the two-stage laser system for aligners according to theinvention.

FIG. 30 is illustrative of one embodiment of the resonator mirrors usedin the amplification-stage laser of FIG. 29.

FIG. 31 is illustrative of a two-stage laser system for aligners as inFIG. 16, wherein the input side mirror and the output side mirror in theresonator in the amplification-stage laser are arranged in nonparallelrelations.

FIG. 32 is illustrative of under what conditions laser output can beeffectively taken while laser light makes a given frequency ofroundtrips in the resonator in the amplification-stage laser.

FIG. 33 is illustrative, as in FIG. 32, of under what conditions laseroutput can be effectively taken while laser light makes a givenfrequency of roundtrips in the resonator in the amplification-stagelaser.

FIG. 34 is illustrative, as in FIG. 32, of under what conditions laseroutput can be effectively taken while laser light makes a givenfrequency of roundtrips in the resonator in the amplification-stagelaser.

FIG. 35 is illustrative of how to calculate the relations between theposition and the angle of injection of seed light at the position of theinput side mirror (rear side mirror) necessary to effectively take laseroutput out of the output side mirror without deviation from thedischarge area.

FIG. 36 is illustrative of the embodiment of FIG. 31 as viewed from theside opposite to the chamber, wherein the edge of the input side mirroris in alignment with the end of the discharge area.

FIG. 37 is illustrative of one area out of which laser output can beeffectively taken in a parallel arrangement of the input side mirror andthe output side mirror, wherein the edge of the input side mirror is inalignment with the end of the discharge area.

FIG. 38 is illustrative of an area out of which laser output can beeffectively taken with the input side mirror inclined.

FIG. 39 is illustrative of why higher laser output and more extendedpulse width are achievable in a nonparallel arrangement than in aparallel arrangement of the input side mirror and the output sidemirror.

FIG. 40 is a view for studying the angle relation of inclination in anonparallel arrangement of two plane mirrors that form together theresonator in the amplification-stage laser.

FIG. 41 is illustrative in schematic of the range for obtaining a properinclination.

FIG. 42 is illustrative of another embodiment wherein seed light isentered as is the case with the two-stage laser system for aligners ofFIG. 25 according to the invention.

FIG. 43 is illustrative of a modification to the input side mirrorusable in the embodiment of FIG. 31.

FIG. 44 is illustrative of the arrangement corresponding to FIG. 31 butwith the use of the input side mirror of FIG. 43.

FIG. 45 is illustrative of a modification to the output side mirrorusable in the embodiment of FIG. 42.

FIG. 46 is illustrative of the arrangement corresponding to FIG. 42 butwith the use of the output side mirror of FIG. 46.

FIG. 47 is illustrative of another arrangement wherein seed light isentered as in FIG. 21 of the two-stage laser system for alignersaccording to the invention.

FIG. 48 is a top view illustrative of one embodiment wherein seed lightis injected in the resonator in the amplification-stage laser from itsside opposite to the laser exit side.

FIG. 49 is a top view illustrative of another embodiment wherein seedlight is injected in the resonator in the amplification-stage laser fromits side opposite to the laser exit side.

FIG. 50 is indicative of the reflection characteristics of CaF₂ toP-polarized light.

FIG. 51 is a top view illustrative of yet another embodiment whereinseed light is injected in the resonator in the amplification-stage laserfrom its side opposite to the laser exit side, and shows theconstruction of the window member in that case.

FIG. 52 is a top view illustrative of a further embodiment wherein seedlight is injected in the resonator in the amplification-stage laser fromits side opposite to the laser exit side, and shows the construction ofthe prism in that case.

FIG. 53 is a top view illustrative of an embodiment corresponding toFIG. 48 but with the injection of seed light from between the outputside mirror and the chamber in the amplification-stage laser.

FIG. 54 is a top view illustrative of an embodiment corresponding toFIG. 49 but with the injection of seed light from between the outputside mirror and the chamber in the amplification-stage laser.

FIG. 55 is a top view illustrative of an embodiment corresponding toFIG. 51 but with the injection of seed light from between the outputside mirror and the chamber in the amplification-stage laser, and showsthe construction of the window member in that case.

FIG. 56 is a top view illustrative of an embodiment corresponding toFIG. 52 but with the injection of seed light from between the outputside mirror and the chamber in the amplification-stage laser.

FIG. 57 is a top view illustrative of one embodiment wherein seed lightis guided directly in the laser chamber in the amplification-stagelaser.

FIG. 58 is a top view illustrative of another embodiment wherein seedlight is guided directly in the laser chamber in the amplification-stagelaser.

FIG. 59 is a top view illustrative of one embodiment wherein seed lightis guided to the amplification-stage laser in the back surface injectionmode.

FIG. 60 is illustrative of relations between the input side mirror (thereflectivity of the rear mirror) and laser output after injection andsynchronization.

FIG. 61 is illustrative of one example indicative of an effectiveenabling area with respect to the angle and position, θin and Xin, ofinjection of seed light in the case where the seed light is injectedfrom the back surface of the input mirror in the input side mirror.

FIG. 62 is a top view illustrative of one embodiment of the mode ofintroducing seed light in the amplification-stage laser via a beamsplitter in the resonator in the amplification-stage laser.

FIG. 63 is a top view illustrative of another embodiment of the mode ofintroducing seed light in the amplification-stage laser via a beamsplitter in the resonator in the amplification-stage laser.

FIG. 64 is a top view illustrative of one embodiment wherein seed lightis injected in the amplification-stage laser through the output sidemirror by way of a beam splitter.

FIG. 65 is illustrative of part of another embodiment of the two-stagelaser system for aligners according to the invention.

FIG. 66 is illustrative of part of yet another embodiment of thetwo-stage laser system for aligners according to the invention.

FIG. 67 is illustrative in schematic of the construction of oneembodiment of the two-stage laser system for aligners according to theinvention, wherein a ring resonator is used in the amplification-stagelaser.

FIG. 68 is illustrative in schematic of the construction of anotherembodiment of the two-stage laser system for aligners according to theinvention, wherein a ring resonator is used in the amplification-stagelaser.

FIG. 69 is illustrative of the principles of why an interference fringepattern occurs in the two-stage laser system for aligners.

FIG. 70 is indicative of relations between the length twice as long asthe resonator length L of the amplification-stage laser and thevisibility of an interference fringe pattern.

FIG. 71 is illustrative in schematic of interference fringes of lightfrom two points in a given share quantity as well as the maximum andminimum fringe intensities.

FIG. 72 is a view, as in FIG. 71, of a laser portion added to it.

FIG. 73 is illustrative of the results of measurement of the visibilityof a line narrowing laser, the results of measurement of visibility inthe case where laser light is amplified in an unstable resonatoramplification-stage laser, and the results of measurement of visibilityin the case where laser light is amplified in a stable resonatoramplification-stage laser.

FIG. 74 is indicative of the section (beam profile) of laser lightemitted out of the oscillation-stage laser.

FIG. 75 is a view for studying coherence between laser light and laserlight spaced away by a different distance in the beam profile.

FIG. 76 is illustrative of how to cut a beam portion out of seed lightfrom the oscillation-stage laser.

FIG. 77 is illustrative of how the post-amplification laser lightdiverges while high coherence is kept intact in the case where the laseris expanded in a prior art amplification-stage laser resonator.

FIG. 78 is illustrative in further detail of how the seed lightexplained with reference to FIGS. 76 and 77 diverges in theamplification-stage laser using an unstable resonator.

FIG. 79 is a view similar to FIG. 76.

FIG. 80 is a view similar to FIG. 77.

FIG. 81 is illustrative in further detail of how the seed lightexplained with reference to FIGS. 79 and 80 diverges in theamplification-stage laser using an unstable resonator.

BEST MODE FOR CARRYING OUT THE INVENTION

First of all, the principles of the two-stage laser system for alignersaccording to the invention are now explained.

As described with reference to FIG. 73, it has been found that in theMOPO system comprising an oscillation-stage laser and anamplification-stage laser in which laser light (seed light) oscillatedout of the oscillation-stage laser is entered for amplification, andwhich comprises a resonator comprising an input side mirror and anoutput side mirror, if the resonator in the amplification-stage laser isconfigured as a stable resonator, it is then possible to achieve a lowspatial coherence equivalent to that of the oscillation-stage laser.

FIG. 1 is illustrative in schematic of the basic arrangement of thetwo-stage laser system for aligners according to the invention. Thetwo-stage laser system for aligners according to the invention is a MOPOsystem as described above, which comprises an oscillation-stage laser(MO: Master Oscillator) 50, and an amplification-stage laser (PO: PowerOscillator) 60 in which seed light oscillated out of theoscillation-stage laser 50 is entered and amplified to produce laserlight. The amplification-stage laser 60 is equipped with a Fabry-Perotetalon resonator comprising an input side mirror (rear side mirror) 1and an output side mirror (front side mirror) 2, between which a chamber3 filled with a laser gas is located. The amplification-stage laser 60further comprises discharge electrodes, etc. for exciting the laser gasin the chamber 3 to form a gain area.

In the oscillation-stage laser 50, typically, a chamber 53 filled with alaser gas is provided in a laser resonator comprising a rear side mirrorthat also serves as an optical element in a line narrowing module 51constructed from, for instance, an expanding prism and a grating(diffraction grating), and a front mirror 52. The oscillation-stagelaser 50 further comprises discharge electrodes, etc. for exciting thelaser gas in the chamber 53 to form a gain area.

Although not essential, between the oscillation-stage laser 50 and theamplification-stage laser 60, there is located a conversion opticalsystem 70 for reducing the sectional area of a seed light beam enteredfrom the oscillation-stage laser 50 in the amplification-stage laser 60or converting the angle of divergence of seed light from theoscillation-stage laser 50.

In the laser system of the invention, if the resonator comprising theinput side mirror 1 and the output side mirror 2 in theamplification-stage laser 60 is constructed of a stable resonator asdescribed above, it is then possible to achieve a low spatial coherenceequivalent to that of the oscillation-stage laser.

The stable resonator in the laser system must satisfy the followingcondition (a):0≦(1−L/R1)(1−L/R2)≦1  (a)Here R1 is the radius of curvature of the input side mirror (rear sidemirror) 1, R2 is the radius of curvature of the output side mirror 2,and L is a mirror-to-mirror spacing, provided that the radius ofcurvature of a concave mirror is defined as positive and the radius ofcurvature of a convex mirror as negative.

Such a stable resonator has a multimode as in the resonator used in theoscillation-stage laser 50, and by using such a stable resonator at theamplification-stage laser 60, it would be possible to achieve a lowspatial coherence equivalent to that of the oscillation-stage laser 50.

However, only with the use of such a stable resonator at theamplification-stage laser 60, seed light is not geometrically magnifiedin the stable resonator, when the amplified laser light leaves thestable resonator, failing to bury the gain area of the laser gas in theamplification-stage laser 60 with seed light for efficientamplification.

Therefore, light having divergence is used as the seed light oscillatedout of the oscillation-stage laser 50 in the invention, and using thedivergence of that seed light, the gain area of the laser gas is buriedwith the seed light that diverges while a plurality of reflections occurin the stable resonator, so that high-efficient amplification can takeplace.

Gas lasers used as light sources in semiconductor aligners, forinstance, fluorine molecule (F₂) laser, KrF excimer laser and ArFexcimer laser make multimode oscillations. In general, the oscillatedlaser light diverges to some degrees, and the use of such a gas laserfor the oscillation-stage laser 50 will allow the seed light enteringthe amplification-stage laser 60 to have divergence. It is noted,however, that the angle of divergence of that seed light could becontrolled through an optical system as described later; the angle ofdivergence of the seed light entering the amplification-stage laser 60could be controlled within the range as desired.

Therefore, the minimum divergence required for the seed light is defineddepending on the pulse width in the amplification-stage laser 60, asfollows.

FIGS. 2(a) and 2(b) are illustrative in vertical and horizontalsections, respectively, of the definition of divergence required for theseed light. As shown, the resonator in an amplification-stage laser 60comprises an input side mirror 1 and an output side mirror 2, betweenwhich there is positioned a chamber 3 filled with a laser gas. In thisembodiment, the laser gas within the chamber 3 is excited by dischargebetween an upper electrode 4 and a lower electrode 5 to form a gainarea. Further between the input side mirror 1 and the output side mirror2, there is located an aperture (opening) 6 that will determine the beamsize of output laser light.

Upon entrance from the input side mirror 1, seed light reflects pluraltimes in the resonator in the amplification-stage laser 60 within theduration of pulse width, so that the gain area (discharge area) can beburied with the seed light having divergence. Therefore, the seed lightof an effective pulse width P in the amplification-stage laser 60 musthave the angles of divergence, θv and θh, in the vertical and horizontaldirections, as defined below. Here, P is the effective pulse width, c isthe velocity of light, L is a resonator length, Vs and Hs are beamdiameters in the vertical and horizontal directions of the seed light,and Va and Ha are beam diameters in the vertical and horizontaldirection of amplified light, provided that each beam diameter ismeasured at a position of a peak strength of 1/e². $\begin{matrix}\begin{matrix}{{\theta\quad h}\quad \geqq {{Tan}^{- 1}\left\lbrack \quad{\left\{ {\left( {{Ha}\quad - \quad{Hs}} \right)/2} \right\} \cdot {\left( {1/L} \right)/\left. \left( {P \cdot {c/L}} \right. \right\rbrack}} \right.}} \\{= {{Tan}^{- 1}\quad\left\{ {\left( {{Ha}\quad - \quad{Hs}} \right)/\left( {2 \cdot P \cdot c} \right)} \right\}}}\end{matrix} & (2) \\\begin{matrix}{{\theta\quad v}\quad \geqq {{Tan}^{- 1}\left\lbrack \quad{\left\{ {\left( {{Va}\quad - \quad{Vs}} \right)/2} \right\} \cdot {\left( {1/L} \right)/\left. \left( {P \cdot {c/L}} \right. \right\rbrack}} \right.}} \\{= {{Tan}^{- 1}\quad\left\{ {\left( {{Va}\quad - \quad{Vs}} \right)/\left( {2 \cdot P \cdot c} \right)} \right\}}}\end{matrix} & (3)\end{matrix}$Here θv and θh are the angles of divergence of laser light entered inthe amplification-stage laser in the vertical and horizontal directions,respectively, P is the effective pulse width, c is the velocity oflight, L is the resonator length, Vs and Hs are the beam diameters oflaser light entered in the amplification-stage laser in the vertical andhorizontal directions, respectively, and Va and Ha are the beam light ofoutput light in the vertical and horizontal directions, respectively.

For instance, when Vs=0 mm, Va=16 mm, L=1,000 mm and the effective pulsewidth (the width of laser pulses actually injected in theamplification-stage laser 60) P=10 ns, seed light having a divergence of1 mrad≦θv will be needed, and when Hs=1 mm, Ha=3 mm, L=1,000 mm and theeffective pulse width P=10 ns, seed light having a divergence of 0.3mrad≦θh will be required.

While, in the above discussions, both the input and output side mirrors1 and 2 of the resonator in the amplification-stage laser 60 are assumedto comprise plane mirrors, yet it is required that the output sidemirror 2 that is the reflecting mirror for reflecting the seed lightfirst be a plane mirror (R2=∞) to bury the gain area using thedivergence of the seed light. On the contrary, the input side mirror 1could be either a plane mirror or a concave mirror included in the rangethat satisfies the above formula (a), that is, the range capable ofmeeting L≦R1.

When the above formulae (2) and (3) are satisfied, narrow-banded laserlight from the oscillation-stage laser 50 fills the gain area (dischargearea) by the divergence effect in the amplification-stage laser 60, asshown in FIGS. 2(a) and 2(b). In the area filled with the laser lightfrom the oscillation-stage laser 50, induction emission takes placewhile keeping the spectral characteristics of the oscillation-stagelaser 50 intact, so that the amplification-stage laser 60 oscillateswith narrow-band laser spectra having similar characteristics to thoseof the oscillation-stage laser 50.

Some embodiments of the construction of the resonator used with theamplification-stage laser 60 are now explained with reference to FIGS.3-8, throughout which (a1) is a front view, (a2) is a longitudinalsection, (a2′) is a cross section and (a3) a rear view as viewed fromthe input side of the input side mirror 1, and (b1) is a front view and(b2) is a sectional view as viewed from the input side of the outputside mirror 2. Notice here that an arrow added to (a2) is indicative ofan input direction of seed light and an arrow added to (b2) isindicative of an output direction of output laser light.

In the embodiment of FIG. 3, the input side mirror 1 is formed of aplane substrate having a seed light-introduction circular hole 7 in itscenter portion. The plane substrate is applied on its (output side) backsurface with a high-reflectivity (total-reflection) mirror coating 8.The (input side) front surface of the plane substrate may or may nothave an antireflection coating 9. The output side mirror 2 is formed ofa plane substrate that is applied on its (input side) front surface witha partial reflecting mirror coating 10 and on its (output side) backsurface with an antireflection coating 11.

Throughout the invention, it is noted that a wedge could be provided onthe seed light input side surface of the input side mirror 1 substrate(to position a wedge surface obliquely, not vertically, with respect toincident light), so that the seed light is reflected at that surface insuch a way as not to go back to the oscillation-stage mirror 50.Likewise throughout the invention, a wedge could be provided on theplane substrate of the output side mirror 2 to keep feeble back-surfacereflection against going back into the resonator.

In the embodiment of FIG. 4, the input side mirror 1 is formed of aplane substrate that has a seed light-introduction longitudinal slit 7′in its center portion. The plane substrate is applied on its (outputside) back surface with a high-reflectivity mirror coating 8. The (inputside) front surface of the plane substrate may or may not have anantireflection coating 9. The output side mirror 2 is formed of a planesubstrate that is applied on its (input side) front surface with apartial reflecting mirror coating 10 and on its (output side) backsurface with an antireflection coating 9. In this embodiment, thelongitudinal slit 7′ is of shape and size substantially equal to thesectional shape of the input side mirror 1 for the seed light from theoscillation-stage laser 50 or seed light deformed by a conversionoptical system 70 interposed between the oscillation-stage laser 50 andthe amplification-stage laser 60.

In the embodiment of FIG. 5, the input side mirror 1 is formed of atransparent plane substrate that is applied on its (output side) backsurface with a high-reflectivity mirror coating 8 except a central,longitudinal area 7″. The (input side) front surface of the planesubstrate may or may not have an antireflection coating 9. The outputside mirror 2 is formed of a plane substrate that is applied on its(input side) front surface with a partial reflecting mirror coating 10and on its (output side) back surface with an antireflection coating 11.In this embodiment, a slit 7″ is provided in the high-reflectivitymirror coating 8 instead of providing the seed light-introduction slit7′ in the substrate (FIG. 4).

In the embodiment of FIG. 6, the input side mirror 1 is constructed asin FIG. 3, and a modification is made to the partial reflecting mirrorcoating 10 of the output side mirror 2. The output side mirror 2 isformed of a plane substrate that has a mirror coating 10′ of relativelylow reflectivity at its (input side) center surface portion, with amirror coating 10 of relatively high reflectivity applied around thecoating 10′. On the portion having the mirror coating 10′ of relativelylow reflectivity in this embodiment, the seed light introduced from acircular hole 7 in the input side mirror 1 is directly incident withoutundergoing repeated reflections between the input side mirror 1 and theoutput side mirror 2; the reflectivity of that portion remainsrelatively low because the amplified laser light is weakened by areduction in the length of the seed light passing through a gain area.

Generally, the partial reflecting mirror coating applied on the outputside mirror 2 has the optimum reflectivity at which laser outputs reacha maximum. As the partial reflecting mirror coating 10 on the outputside mirror 2 is allowed to have the optimum reflectivity, satisfactorylaser output efficiency might be obtained. As discussed above, however,there is a decrease in energy at the center of output beam shapedepending on the distance where the seed light obtains gains, which willotherwise cause the section of laser light (discharge direction) to havean uneven output profile.

In the coating on the output side mirror 2 according to this embodiment,the mirror coating 10′ at the center of the output side mirror 2 and themirror coating 10 applied around it are varied in reflectivity in such away as to obtain as uniform an output profile as possible. This workssomewhat against laser output efficiency, because both the coatings haveoften difficulty in having the optimum reflectivity at which laseroutputs reach a maximum. However, the output profile across the sectionof an output laser beam becomes satisfactory.

In the above embodiment, by way of illustration but not by way oflimitation, the reflectivity of the mirror coating 10′ at the center ofthe output side mirror 2 is set lower than that of the mirror coating10′ around it.

As a matter of course, the reflectivity at which the above maximum laseroutput is not obtained is higher or lower than the above optimumreflectivity. In other words, the mirror coating has a plurality ofreflectivities at which there are obtained given laser outputs lowerthan the above maximum laser outputs. In the above embodiment,therefore, even when the reflectivity of the mirror coating 10′ at thecenter of the output side mirror 12 is set higher than that of themirror coating 10 applied around it, effects equivalent to those of theabove embodiments will often be obtained.

In the embodiments of FIGS. 7 and 8, the input and output side mirrors 1and 2 are constructed as in FIG. 4. However, the (output side) backsurface of the substrate of the input side mirror 1 has a cylindricalconcave shape rather than a planar shape. In FIG. 7, the generating lineof that cylindrical concave surface directs vertically (in thelongitudinal direction of the longitudinal slit 7′), and in FIG. 8, thegenerating line directs horizontally (in the direction vertical to thelongitudinal direction of the longitudinal slit 7′). In either case, theradius of curvature of that cylindrical concave surface is set in such away as to satisfy the above condition L≦R1.

Specifically but not exclusively, FIGS. 3-8 are illustrative of mirrorarrangements for the resonator used with the amplification-stage laser60. Combinations of them or modifications to them could be used. Forinstance, the surface of the input side mirror 1 to be provided with thehigh-reflectivity mirror coating 8 could be configured in a sphericalconcave form.

A partial reflecting mirror coating could be provided all over theoutput side surface of the input side mirror 1. In this case,fabrication could be facilitated because of no need of providing such acircular hole 7 as depicted in FIG. 3 or such a slit 7″ provided withthe antireflection coating 9 as depicted in FIG. 5. It is noted,however, that there is a lowering in the efficiency of utilization ofthe seed light, because a part of the seed light is reflected uponincidence and is not injected in the amplification-stage laser 60.

The conversion optical system 70 interposed between theoscillation-stage laser 50 and the amplification-stage laser 60 is nowexplained. As already described, this conversion optical system 70 isprovided if required, and has primarily both or either one of twofunctions.

Generally, as the energy density of the seed light entering theamplification-stage laser 60 becomes too low, it is difficult to obtainany sufficient amplification factor at the amplification-stage laser 60.In that case, it is desired that the conversion optical system 70 beprovided such that the beam diameter of the seed light is reduced toincrease the energy density before the seed light enters theamplification-stage laser 60, as depicted in FIG. 9.

Such a seed light beam reduction optical system uses such beam diameterreduction prisms 71 and 72 as depicted in FIG. 10. In this case, eachprism 71, 72 comprises a rectangular refracting prism wherein as inputlight enters vertically an entrance side plane, it passes almostvertically through the entrance side plane and is refracted at an exitside oblique surface, so that the section of the beam in the paper isreduced. With a plurality of, preferably, an even number of, such prisms71 and 72, therefore, the beam diameter of the seed light in aone-dimensional direction (longitudinally or transversely) or atwo-dimensional direction (longitudinally or transversely) can bereduced to increase the energy density.

Alternatively, the seed light beam reduction optical system could usesuch a telephoto optical system as depicted in FIGS. 11(a) and 11(b). Inthe optical system of FIG. 11(a), a positive lens 73 having a longerfocal length and a positive lens 74 having a shorter focal length arelocated at a co-focal point, enabling the beam diameter to be reduced bythe focal length ratio. In the optical system of FIG. 11(b), a positivelens 75 having a longer focal length and a negative lens 76 having ashorter focal length are again located at a co-focal point. In thiscase, too, the beam diameter is reduced by the focal length ratio(absolute value).

One function of the conversion optical system 70 is to reduce the beamdiameter of such seed light as mentioned above, thereby increasing theenergy density, and another function is to enlarge the angles ofdivergence, θv and θh, of the seed light entering theamplification-stage laser 60 in such a way as to satisfy formulae (2)and (3) in the case where the divergence of the seed light oscillatedout of the oscillation-stage laser 50 does not satisfy formulae (2) and(3). To perform the second function, i.e., to tweak the angles ofdivergence, θv and θh, of the seed light, such a telephoto opticalsystem as depicted in FIGS. 11(a) and 11(b) is used to adjust thedistance between the positive lenses 73 and 74, and the distance betweenthe positive lens 75 and the negative lens 76.

By the way, gas lasers used as light sources for semiconductor aligners,for instance, fluorine molecule (F₂) laser, KrF excimer laser and ArFexcimer laser, are excited by discharge between discharge electrodes 54and 55 to form a gain area; the section of the seed light from theoscillation-stage laser 50 has a longitudinally slender shape (becausethe discharge electrodes 54 and 55 are provided to sandwich it fromabove and below). When the seed light has a longitudinally slender shapein section, its horizontal direction divergence is likely to satisfy therelation of formula (2); however, its vertical direction divergencebecomes small, often failing to satisfy the relation of formula (3). Inthat case, a negative cylindrical lens 77 having a cylindrical concavesurface 78 with the generating line in the horizontal direction,negative refracting power in the vertical direction alone, divergence inthe vertical direction and no refracting power in the horizontaldirection, as shown in the three-view drawing of FIG. 12(b), is providedas the conversion optical system 70 to be interposed between theoscillation-stage laser 50 and the amplification-stage laser 60, asdepicted in FIG. 12(a), thereby enabling both the angles of divergence,θv and θh, of the seed light entering the amplification-stage laser 60in the vertical and horizontal directions to satisfy the relations ofthe above formulae (2) and (3).

One exemplary construction of the two-stage laser system for alignerscomprising the essential features of the invention is now explained withreference to FIG. 13 showing the generation construction thereof.

When the MOPO system according to the invention is a fluorine molecule(F₂) laser system, a chamber 53 in the oscillation-stage laser 50 and achamber 3 in the amplification-stage laser 60 are each filled with alaser gas comprising fluorine (F₂) gas and buffer gas comprising helium(He), neon (Ne) or the like. When the MOPO system is a KrF excimer lasersystem, the chamber 53 in the oscillation-stage laser 50 and the chamber3 in the amplification-stage laser 60 are each filled with a laser gascomprising krypton (Kr) and fluorine (F₂) gases and buffer gascomprising helium (He), neon (Ne) or the like. When the MOPO system isan ArF excimer laser system, the chamber 53 in the oscillation-stagelaser 50 and the chamber 3 in the amplification-stage laser 60 are eachfilled with a laser gas comprising argon (Ar) and fluorine (F₂) gasesand buffer gas comprising helium (He), neon (Ne) or the like. The laserchamber 53 in the oscillation-stage laser 50 has a discharge portioncomprising a pair of discharge electrodes 54 and 55, and the laserchamber 3 in the amplification-stage laser 60 has a discharge portioncomprising a pair of discharge electrodes 4 and 5. These dischargeportions have a pair of cathodes 55, 5 and a pair of anodes 54, 4located vertically in a parallel direction to the paper. High voltagepulses are applied to these pairs of electrodes 54 and 55, and 4 and 5from the associated power sources 56 and 16, thereby producingdischarges between these electrodes.

At both ends of axial extension from the pairs of electrodes 54 and 55,and 4 and 5 in the chambers 53 and 3 in the oscillation-stage laser 50and amplification-stage laser 60, there are located window members 57and 17, each formed of a material transparent to laser oscillated lightsuch as CaF₂. Exposed surfaces of the window members 57 and 17 oppositeto the interiors of the chambers 53 and 3 are located parallel with eachother and at such a Brewster angle with respect to laser light as toreduce reflection losses. The window members 57 and 17 are alsopositioned in such a way as to place the P-polarized component of laserlight in the horizontal direction.

A cross-flow fan, not shown in FIG. 13, is housed in each chamber 53, 3to circulate the laser gas therein and send the laser gas into thedischarge portion. Each of the oscillation-stage laser 50 and theamplification-stage laser 60 comprises a F₂ gas supply system and abuffer gas supply system for supplying F₂ gas and buffer gas to thechamber 53, 3, and a gas evacuation system for evacuating the laser gasin the chamber 53, 3. In FIG. 13, these are collectively designated as agas supply/evacuation control valve 58 and a gas supply/evacuationcontrol valve 18. Notice that the KrF laser system and the ArF lasersystem comprise a Kr gas supply system and an Ar gas supply system,respectively. The gas pressures in the chambers 53 and 3 are monitoredby pressure sensors P1 and P2, respectively, and gas pressureinformation is sent to a utility controller 81 that controls the gassupply/evacuation valves 58 and 18, thereby controlling the gascomposition and pressure in the oscillation-stage chamber 53 and theamplification-stage chamber 3, respectively.

Laser outputs change with gas temperatures. To this end, gas temperaturecontrol is carried out. The gas temperature is monitored by temperaturesensors T1 and T2 added to the respective chambers 53 and 3, andtemperature signals are sent to the utility controller 81 that controlsthe flow of coolant water by coolant water flow control valves 59 and19, respectively. As a result, the amounts of exhaust heat in therespective heat exchangers 34 and 44 in the chambers 53 and 3 arecontrolled for temperature control.

The oscillation-stage laser 50 comprises a line narrowing module (LNM)51 constructed from an expanding prism and a grating (diffractiongrating), and a laser resonator is constructed of an optical element inthe line narrowing module 51 and a front mirror 52. Although not shown,a line narrowing module using etalon and a total-reflection mirrorinstead of the expanding prism and grating could be used.

A part of laser light emitted out of the oscillation-stage laser 50 andthe amplification-stage laser 60 is split by means of a laser splitternot shown and guided to monitor modules 34 and 45, respectively, whichmonitor the laser light characteristics of the oscillation-stage laser50 and the amplification-stage laser 60, respectively, such as outputs,line widths and center wavelengths. In FIG. 13, the monitor modules 35and 45 are installed in both the oscillation-stage laser 50 and theamplification-stage laser 60, although it is acceptable to use eitherone of them.

Center wavelength signals from the monitor modules 35 and 45 are sent toa wavelength controller 82 that drives the optical element in the linenarrowing module 51 through a driver 83 to make a selection fromwavelengths and performs wavelength control in such a way that thecenter wavelength of the oscillation-stage laser 50 becomes the desiredwavelength. Notice that the above wavelength control could be carriedout by issuing commands from the wavelength controller 82 to the driver83 on the basis of wavelength information from the monitor module 45 towhich a part of laser light emitted out of the amplification-stage laser60 is guided in such a way that the wavelength of laser light emittedout of the oscillation-stage laser 50 becomes the given wavelength.

Laser output signals from the monitor modules 35 and 45 are sent to anenergy controller 84. Then, applied voltage is controlled by way of asynchronous controller 85, and control is done in such a way that theenergy of the oscillation-stage laser 50 and the amplification-stagelaser 60 has the desired value. The output signals of the monitor module45 could be sent to the energy controller 84 as shown at (1) in FIG. 13,or alternatively outputs at an output monitor provided on the side of asemiconductor aligner 100, although not shown, could be supplied to theenergy controller 84 as shown at (2).

After passing through the monitor module 35, the laser light (seed laserlight) from the oscillation-stage laser 50 passes through a beamsteering unit 86 comprising a reflecting mirror, etc. and then throughthe conversion optical system 70. Then, the laser light is guided to theamplification-stage laser 60 for injection. The conversion opticalsystem 70 comprises a mechanism wherein, as previously described, theangle of divergence of the laser light from the oscillation-stage laser50 is controlled to such a value as to allow the oscillation-stage laserlight to be injected in the amplification-stage laser 60 at a givenangle of divergence. With the MOPO system of the invention, the stableresonator made up of an input side mirror (rear side mirror) 1 and anoutput side mirror (front side mirror) 2 is used on theamplification-stage laser 60 in such a way that amplification could takeplace even with a limited input. The input side mirror 1 is holed at 7(FIG. 3). Passing through the hole 7, the laser is reflected as shown byan arrow in FIG. 13, and the injected seed laser light is expanded bythe hole, passing effectively through the discharge portion to increasethe power of the laser light. Finally, the laser leaves the output sidemirror 2.

Instead of providing a spatial opening for the hole 7 in the input sidemirror 1, it is acceptable to use a mirror substrate with only a holeportion applied with an antireflection coating (see FIG. 5).

By way of a power source 56 built up of a charger 31/switch 32/MPC(magnetic pulse compression circuit) 33 and a power source 16 built upof a charger 41/switch 42/MPC (magnetic pulse compression circuit) 43,high voltage pulses are applied to a pair of discharge electrodes 54 and55 in the oscillation-stage laser 50 and a pair of discharge electrodes4 and 5 in the amplification-stage laser 60, respectively, to give riseto discharge between the electrodes 54 and 55 and between the electrodes4 and 5. This discharge in turn causes excitation of the laser gasesfilled in the laser chambers 53 and 3, respectively.

At the respective power sources 56 and 16, capacitors are charged by thechargers 31 and 41. As the switches 32 and 42 are held on, energycharged in the capacitors is transferred as voltage pulses to themagnetic pulse compression circuits 33 and 43 where they are compressedfor application to the pair of electrodes 54 and 55 and the pair ofelectrodes 4 and 5. Although not shown, the power sources 56 and 16 areeach provided with a step-up transformer that could be used to boost upvoltage pulses.

The switches 32 and 42 are put on or off in response to operatingcommands (trigger signals) from the synchronous controller 85.

The synchronous controller 85 sends trigger signals to the power source56 built up of a charger 31/switch 32/MPC (magnetic pulse compressioncircuit) 33 and the power source 16 built up of a charger 41/switch42/MPC (magnetic pulse compression circuit) 43 such that discharge isgenerated at the amplification-stage laser 60 at a timing of injectingthe laser emitted out of the oscillation-stage laser 50 in theamplification-stage laser 60. As there is a discharge timing lag betweenthe oscillation-stage laser 50 and the amplification-stage laser 60, thelaser light emitted out of the oscillation-stage laser 50 will be notefficiently amplified. The synchronous controller 85 gleans informationabout when discharge starts to occur at the oscillation-stage laser 50and the amplification-stage laser 60 from discharge sensors 36 and 46,respectively, and laser output information from the energy controller 85to set a delay time between the trigger signals sent to the power source56 for the oscillation-stage laser 50 and the trigger signals sent tothe power source 16 for the amplification-stage laser 60.

The utility controller 81, energy controller 84 and wavelengthcontroller 82 are connected to the main controller 80. The maincontroller 80 is connected to the aligner 100. In response to commandsfrom the aligner 100, the main controller 80 is operable to allocate therespective controls to the respective controllers 81, 84 and 82,allowing the respective controllers 81, 84 and 82 to execute therespective controls.

The laser light emitted out of the oscillation-stage laser 50 is alignedby the beam steering unit 86 made up of two mirrors such that it passesthrough a discharge area in the amplification-stage laser 60. The twomirrors that form the beam steering unit 86 are driven by a driver 87for angle control, so that the direction of travel of the laser lightissuing from the oscillation-stage laser 50 is controlled.

How to control the beam steering unit 86 is now specifically explained.For instance, suppose here that the direction of travel of laser lightgiven out of the oscillation-stage laser 50 was not aligned such that itpassed through the discharge area in the amplification-stage laser 60. Apart or the whole of the laser light emitted out of theoscillation-stage laser 50 will then be cut off or reflected in anundesired direction, for instance, by the discharge electrodes 4 and 5in the amplification-stage laser 60, failing to leave theamplification-stage laser 60 or causing laser power to become smallerthan the desired value. To avert this, the beam steering unit 86 iscontrolled in such a way as to have a maximum laser light output whilethat output is monitored on the monitor module 45. This is explainedwith reference to FIG. 13. The results of monitoring by the monitormodule 45 are sent to the wavelength controller 82 that, on the basis ofoutput results received from the monitor module 45, gives a command tothe driver 87 to drive and control the beam steering unit 86 such thatthe output reaches a maximum, thereby controlling the direction oftravel of the laser light issuing from the oscillation-stage laser 50.

Part of one specific embodiment of the two-stage laser system foraligners of such construction is shown in FIG. 14. FIGS. 14(a) and 14(b)are an top view and a side view of that part. An ArF excimer laser of193 nm wavelength is used for each of the oscillation- andamplification-stage lasers 50 and 60, and a resonator built up of aplanar input side mirror 1 and a planar output side mirror 2 as shown inFIG. 3 is used in the amplification-stage laser 60. By way of the beamsteering unit 86 made up of two mirrors, the seed light from theoscillation-stage laser 50 is injected in the input side mirror 1 with ahole 7 in the resonator in the amplification-stage laser 60. The inputside mirror 1 is applied on its reflecting surface or its chamber 3 sidesurface with a total-reflection coating, and the output side mirror 2 isa partial reflecting mirror. The hole 7 has a diameter of about 2 mm(=Vs=Hs), the amplification-stage laser 60 has a discharge size of Ha=16mm and Va=3 mm, and the resonator length L is about 1 m. The effectiveinjection pulse width of the oscillation-stage laser 50 is P=20 ns.

With such an arrangement, there were obtained data of such low coherence(share quantity versus visibility relations) as shown in FIG. 15. FIG.15 shows not only the share quantity versus visibility relationsobtained from the oscillation-stage laser 50 (oscillator) in thisembodiment (using the resonator of the invention; similar to FIG. 73)but also the share quantity versus visibility relations in the case ofusing a prior art unstable resonator.

Under the above conditions, the angle of divergence in the horizontaldirection, θh, must satisfy the requirement of 0.05 mrad<θh, and theangle of divergence in the vertical direction, θv, must satisfy therequirement of 1.2 mrad<θv. In the oscillation-stage laser 50 accordingto the above embodiment, the angle of divergence in the horizontaldirection, θh, is 1 mrad and the angle of divergence in the verticaldirection, θv, is 3 mrad; they satisfy the above conditions (2) and (3).For this reason, any conversion optical system 70 is not used in thisembodiment.

From these results, it has been found that low coherence equivalent tothat in a prior art MOPA laser system is achievable while maintainingline widths and energy stability comparable to those in the prior artMOPA laser system using an unstable resonator.

By the way, the introduction of the seed light from theoscillation-stage laser 50 in the resonator in the amplification-stagelaser 60 are achievable by the hole 7, 7′ provided in the center portionof the input side mirror 1, the slit 7″ formed in the center portion ofthe high-reflectivity mirror coating 8 applied on the input side mirror1, and a partial reflecting mirror coating applied all over the inputside surface of the input side mirror 1, as described with reference toFIGS. 3-8. However, there are available some other modifications, asdescribed below.

FIG. 16 is illustrative of part of one exemplary modification. FIG.16(a) is a top view, FIG. 16(b) is a side view, and FIG. 16(c) is a viewof the input side mirror 1 in the amplification-stage laser 60, asviewed from its chamber 3 side. In the input side mirror 1 in theamplification-stage laser 60 according to this modification, twohigh-reflectivity, rectangular plane mirrors 1 ₁ and 1 ₂ are arrangedside by side on the same plane with a gap 21 between their edges. Thetwo plane mirrors 1 ₁ and 1 ₂ are located such that the gap 21 isnarrower than a discharge area 22 formed by discharge electrodes 4 and 5in the amplification-stage laser 60. In other words, the seed light 23is introduced through the slit 21 formed between the two plane mirrors 1₁ and 1 ₂. The high-reflectivity mirror planes of the two plane mirrors1 ₁ and 1 ₂ lie within the same plane, and so the two plane mirrors 1 ₁and 1 ₂ have the same function as one mirror having a slit. In thisembodiment, the same function that the input side mirror 1 of FIG. 4 hasis achieved by use of two high-reflectivity mirrors 1 ₁ and 1 ₂. Evenwith such an arrangement, it is possible to obtain line widths andenergy stability equivalent to those of a prior art MOPO system using anunstable resonator and low coherence comparable to that in a prior artMOPA laser system.

In the input side mirror 1 of FIG. 16, the two high-reflectivity(total-reflection), rectangular plane mirrors 1 ₁ and 1 ₂ are arrangedside by side on the same plane with the gap 21 formed between theiredges. However, the longitudinal slit 7′ formed in the center portion ofthe input side mirror 1, as shown in FIG. 5, could be configured as thesame slit that forms the gap 21 in the embodiment of FIG. 17, as shownin FIGS. 17 and 18.

In FIGS. 17 and 18, (a) is a front view of the input side mirror 1 asviewed from its output side (the chamber 3 side), (b) is a longitudinalsection, and (c) and (d) are illustrative of in what position relationthe input side mirror 1 is located with respect to a discharge area 22,as viewed from the output side of the input side mirror 1.

In the embodiment of FIGS. 17 and 18, the input side mirror 1 is formedof a CaF₂ or other plane substrate transparent to ultraviolet light. Theoutput side surface (FIG. 17(a)) of the plane substrate is applied witha high-reflectivity (total-reflection) mirror coating 8 except a centralslit-form area and a peripheral edge, and an antireflection coating 9 isapplied on a slit-form area 21 at the center portions of the input andoutput side surfaces and on the peripheral edges thereof. FIG. 17 showsan example of the plane substrate having a rectangular shape, and FIG.18 shows an example of the plane substrate having a circular shape.

With the input side mirror 1 of FIG. 16 (FIG. 16(c)), there isdifficulty in the application of coating as far as the mirror end faces(peripheral portions) for the purpose of holding the mirror during vapordeposition. In addition, it is not easy to process the ends of a CaF₂ orother substrate into right-angle faces with high precision; usually,there is a chip off the ends during fabrication. Without chip-freeapplication of a high-reflectivity (total-reflection) coating 8 as faras the ends, the substrate ends having a decreased reflectivity willcause losses leading to an oscillation efficiency drop.

If such an input side mirror 1 as shown in FIGS. 17 and 18 is used withthe laser system shown in FIGS. 16(a) and 16(b), on the other hand, itwill be easy to process the ends of the high-reflectivity(total-reflection) coating 8; it will be possible to apply thehigh-reflectivity coating 8 as far as the boundary between the seedlight 23 and the amplified laser light in the amplification-stage laser60.

The size of the input side mirror 1 shown in FIGS. 17 and 18 shoulddesirously be such that, as shown in FIGS. 17(c) and 17(d) and FIGS.18(c) and 18(d), the longitudinal length of the center slit-form area 21applied with the antireflection coating 9 is longer than the dischargearea 22 defined by the discharge electrodes 4 and 5 in theamplification-stage laser 60.

As shown, the input side mirror 1 is located externally of the laserchamber 3. Thus, even when the distance between the discharge electrodes4 and 5 is designed to become longer, the laser light from the laserchamber 3 is unlikely to lie off the input side mirror 1 as long as thatdistance is within the range of the longitudinal size of the centerslit-form area 21 applied with the antireflection coating 9.

FIGS. 17(c) and 18(c) show that the longitudinal direction of the centerslit-form area 21 applied with the antireflection coating 9substantially matches that of the discharge area 22 defined by thedischarge electrodes 4 and 5 in the amplification-stage laser 60, andFIGS. 17(d) and 18(d) illustrate that the longitudinal direction of thecenter slit-form area 21 applied with the anti-reflection coating 9 issubstantially orthogonal to that of the discharge area 22 defined by thedischarge electrodes 4 and 5 in the amplification-stage laser 60.

In the input side mirror 1 shown in FIGS. 17 and 18, it is noted thatthe slit-form area 21 at the end faces of the input- and output-sidesurfaces as well as the peripheral portion of the plane substrate maynot have the antireflection coating 9. With such an arrangement whereinonly two sites are provided with the high-reflectivity(total-reflection) coating 8 and there is no antireflection coating 9,the robustness of the input side mirror 1 to laser light can be improveddue to no possibility of any coating deterioration.

In the exemplary laser system of FIG. 16, the total-reflection mirror 1(FIG. 16(c), FIG. 17 and FIG. 18) and the partial reflecting mirror 2are each formed of a plane mirror; however, the invention is not alwayslimited to them as long as the stable resonator is set up by bothmirrors 1 and 2.

FIG. 19 is illustrative of part of another embodiment as in FIG. 16. Inthis embodiment, the input side mirror 1 in the amplification-stagelaser 60 is built up of one high-reflectivity (total-reflection),hole-free plane mirror. One such input side mirror 1 is decentered inthe horizontal direction with respect to the seed light from theoscillation-stage laser 50, and located such that its edge is positionedwithin or near the discharge area 22 defined by the discharge electrodes4 and 5 in the amplification-stage laser 60. The seed light 23 isintroduced in the amplification-stage laser 60 from outside along thatedge. With this arrangement, it is possible to prevent pits fromoccurring in the profile of laser light produced out of theamplification-stage laser 60 (there are spots of weak intensity in acenter beam portion). Notice that the optical axis of the seed light 23could be slightly inclined with respect to the optical axis of theinput- and output-side mirrors 1 and 2 in such a way as to fill thedischarge area with the seed light.

As a result of experimentation, the inventors have now found that ifseed light is injected in the amplification-stage laser 60 in such a wayas to fill the discharge area therewith while the optical axis C of theseed light 23 is slightly inclined with respect to the optical axis D ofthe input- and output-side mirrors 1 and 2, it is then possible toobtain much lower coherence so that the efficiency of amplification andoscillation at the amplification-stage laser 60 can be much moreenhanced.

A possible reason for this is now explained with reference to FIG. 20.FIG. 20 is indicative of the principles of how the amplification-stagelaser 60 operates upon entrance of the seed light 23 from the end of thedischarge area 22 while the optical axis C of the seed light 23 isslightly inclined with respect to the optical axis D of the input- andoutput-side mirrors 1 and 2. FIGS. 20(a) and 20(b) are a top view and aside view of the resonator in the amplification-stage laser 60,respectively.

As shown in the top view of FIG. 20(a), the narrow-banded seed light 23leaving the oscillation-stage laser 50 (see FIG. 19) passes through theend of the input side mirror (total-reflection mirror) 1, and isinjected in the amplification-stage laser 60 from the side of thedischarge area 22. This seed light 23 enters the discharge area 22 whileits optical axis C is at a slight angle α (of e.g., about 0.5 mrad) withrespect to the optical axis of the resonator in the amplification-stagelaser 60, and is amplified through the discharge area 22, entering theoutput side mirror (partial reflecting mirror) 2. A part of laser lightamplified upon entrance in the output side mirror 2 is produced as laserlight K1 after passing through the output side mirror 2. Another part oflaser light amplified upon entrance in the output side mirror 2 isreflected by the output side mirror 2.

This reflected light again passes through the discharge area 22 foramplification, and then goes back to the discharge area foramplification after entering the input side mirror 1 and reflectionthereat. The amplified laser light enters the output side mirror 2, anda part of it is produced as laser light K2 after passing through itwhile another is reflected back to the discharge area. After suchresonation is repeated, laser light K3 is produced as the output of theamplification-stage laser 60. Here the angle of incidence of the seedlight 23 on the output side mirror 2 and the angles of incidence andreflection of the amplified light on and at the input- and output-sidemirrors 1 and 2 are indicated by a with respect to the optical axis D ofthe resonator in the amplification-stage laser 60. In this connection,FIG. 20(a) is also illustrative in schematic of the intensitydistributions of output laser light K1, K2 and K3.

In this way, the seed light 23 is subjected to multiple reflectionsbetween the output side mirror (partial reflecting mirror) 2 and theinput side mirror (total-reflection mirror) 1 in a zigzag fashion, asshown in the top view of FIG. 20(a). This state will give birth toeffects equivalent to the case where a plurality of point light sources(S1, S2 and S3) are provided to the output side mirror 2. Spatialcoherence becomes low with increasing light source size. Consequently,with the optical axis C of the seed light 23 at a slight angle withrespect to the optical axis D of the input- and output-side mirrors 1and 2, the amplification and oscillation of laser light having lowspatial coherence could be possible in the amplification-stage laser 60.

With the resonator in the amplification-stage laser 60 shown in FIG. 20,there are mutual misalignments in the exit positions of output laserlight K1, K2 and K3 (in the embodiment of FIG. 20, such misalignmentsoccur at a given spacing of, e.g., about 1 mm, in the horizontaldirection), and so the profile (energy distribution) of the laser lightleaving the output side mirror 2 comes close to a top hat form (arectangular wave-form distribution), allowing the energy density withina laser light plane to become lower than that of a Gaussian beam. As aresult, it is possible to reduce damages to the optical elements in theamplification-stage laser 60 (such as window member 17, input sidemirror 1 and output side mirror 2) as well as to optical elements forshaping the laser beam leaving the amplification-stage laser 60 (such asa total-reflection mirror, a beam expander or the like located in a beamdelivery unit for connecting together the two-stage laser system foraligners and an aligner).

Throughout the invention, the angle of inclination, α (in rad), of theoptical axis D of the resonator in the amplification laser 60 withrespect to the optical axis C of the seed light 23 should desirouslysatisfy the relation of the following condition:0.0005≦2αL≦0.0015  (4)Here L is the length of the resonator in the amplification-stage laser60.

Although described in detail later, it is preferable that the opticalpath difference due to the resonator in the amplification-stage laser 60(an optical path difference between laser light K1 and K2 or between K2and K3) is set longer than the time-based coherent length correspondingto the spectral line width of the narrow-banded seed light 23 producedout of the oscillation-stage laser 50, because laser light K1, K3 and K3do not interfere one another with the result that there are nointerference fringes on the beam profile of the laser light produced outof the amplification-stage laser 60. This in turn leads to not onlyimprovements in the symmetry of the beam profile of the output laserbeam but also reductions in its fluctuations. Thus, it is possible toprovide uniform illumination to masks in the aligner and the subjects tobe exposed to light (e.g. wafers).

Further, if the seed light 23 is injected in the discharge area 22 whileits optical axis C is slightly inclined with respect to the optical axisD of the resonator in the amplification-stage laser 60 as describedabove, then the discharge area 22 in the amplification-stage laser 60can then be filled in it with the seed light 23 or its amplified lighteven at a small angle of divergence of the seed light 23. This in turnallows for the oscillation of the amplification-stage laser 60 byamplified resonation.

In this embodiment, the input- and output-side mirrors 1 and 2 are eachformed of a plane mirror; however, the invention is not necessarilylimited to them as long as the stable resonator is made up of bothmirrors. For instance, if the input side mirror 1 or the output sidemirror 2 is formed of a cylindrical concave mirror, further reductionsof spatial coherence are then possible. That is, as the cylindricalconcave mirror is located such that the generating line directionsubstantially matches the center axis of the discharge direction, itgives rise to a lot more resonance modes, resulting in furtherreductions of spatial coherence in the vertical direction to thedischarge direction.

FIG. 21 is illustrative of part of yet another embodiment, as in FIG.16. In this embodiment, the input side mirror 1 in theamplification-stage laser 60 is made up of one high-reflectivity(total-reflection), hole-free plane mirror. One such input side mirror 1is decentered in the vertical direction to the seed light from theoscillation-stage laser 50 or upward in FIG. 21. The seed light 23 isthen introduced in the discharge area from outside along an edge of thedecentered input side mirror 1 that lies on the opposite side withrespect to the direction of decentration. With this arrangement, it ispossible to prevent pits from occurring on the profile of laser lightproduced out of the amplification-stage laser 60 (there are spots ofweak intensity in a center beam portion). It is acceptable to fill thedischarge area with the seed light 23 while the optical axis of the seedlight 23 is slightly inclined with respect to the optical axis of theinput- and output-side mirrors 1 and 2. As previously stated, thisarrangement allows for a lot lower coherence, so that the amplificationand oscillation of the amplification-stage laser 60 can occur withefficiency.

In the embodiments of FIG. 19 and FIG. 21, the input side mirror 1 ismade up of one high-reflectivity (total-reflection), hole-free planemirror 1. However, it is noted that another input side mirror 1 could beachieved by applying an antireflection coating 9 to a seedlight-incident area of the output side surface of a CaF₂ or other planesubstrate transparent to ultraviolet light and a high-reflectivity(total-reflection) mirror coating 8 to the remaining area, as shown inFIG. 22, FIG. 23 and FIG. 24, respectively, with (a) being a view asviewed from the chamber 3 side and (b) being a sectional view. With eachof the input side mirrors 1 shown in FIGS. 16-21, there is difficulty inapplying coating as far as the ends of the mirror for the purpose ofkeeping the mirror during vapor deposition. It is also not easy toprocess the ends of the CaF₂ or other substrate into right-angle faceswith high accuracy. Usually, there is a chip off the ends duringfabrication. Without chip-free application of a high-reflectivity(total-reflection) coating 8 as far as the ends, the substrate endshaving a decreased reflectivity will cause losses leading to anoscillation efficiency drop. If each of such input side mirrors 1 asshown in FIG. 22, FIG. 23 and FIG. 24 is used, processing of the ends ofthe high-reflectivity (total-reflection) coating 8 will then befacilitated, so that the high-reflectivity coating 8 will be applied asfar as the boundary between the seed light 23 and the amplified laserlight in the amplification-stage laser 60.

Referring here to FIGS. 23(c) and 23(d), there is shown in what relationthe input side mirror 1 is positioned with respect to the discharge area22 in the two-stage laser system shown in FIG. 19 or FIG. 21.Specifically, FIGS. 23(c) and 23(d) are views as viewed from the outputside of the input side mirror 1 (the side on which the seed light 23 isincident). FIG. 23(c) shows that the direction of the end of an areaapplied with a high-reflectivity (total-reflection) coating 8 on theside of the mirror, which is not its peripheral edge side, substantiallymatches the longitudinal direction of the discharge area 22 defined bythe discharge electrodes 4 and 5, and FIG. 23(d) shows that thedirection of the end of the area applied with the high-reflectivity(total-reflection) coating 8 on the side of the mirror, which is not itsperipheral edge side, is substantially orthogonal to the longitudinaldirection of the discharge area 22 defined by the discharge electrodes 4and 5. In FIGS. 23(c) and 23(d), it is preferable that the area ratio ofan area X where the area of the input side mirror 1 applied with thehigh-reflectivity (total-reflection) coating 8 makes an intersectionwith the discharge area 22 and an area Y where the area of the inputside mirror 1 applied with the antireflection coating 9 makes anintersection with the discharge area 22 is at least X<Y. The reason isthat when X>Y, the light oscillated from the amplification-stage laser60 goes back to the oscillation-stage laser 50, doing damage to theoptical elements in the oscillation-stage laser 50 (especially the frontmirror 52), and bring about a drop of the laser output produced out ofthe front mirror 52 in the oscillation-stage laser 50 (the output of theseed light 23).

The size of the input side mirror 1 shown in FIG. 22, FIG. 23 and FIG.24 should desirously be such that the length of the end of the areaapplied with the high-reflectivity (total-reflection) coating 8 on theside of the mirror, which is not its peripheral edge side, is longerthan the length of the discharge area 22 defined by the dischargeelectrodes 4 and 5 in the amplification-stage laser 60 in itslongitudinal direction.

As shown, the input side mirror 1 is located externally of the laserchamber 3. Thus, even when the distance between the discharge electrodes4 and 5 is designed to become longer, the laser light from the laserchamber 3 is unlikely to lie off the input side mirror 1 as long as thatdistance is within the range of the length of the end of the areaapplied with the high-reflectivity (total-reflection) coating 8 and theantireflection coating 8 on the side of the mirror, which is not itsperipheral edge side.

In the input side mirror 1 shown in FIG. 22, FIG. 23 and FIG. 24,portions of the plane substrate other than its portion applied with thehigh-reflectivity (total-reflection) coating 9 may not be applied withthe anti-reflection coating 8. With such an arrangement wherein only onesite is provided with the high-reflectivity (total-reflection) coating 8and there is no antireflection coating 9, the robustness of the inputside mirror 1 to laser light can be improved due to no possibility ofany coating deterioration.

In the exemplary two-stage laser system as described above, when theseed light 23 from the oscillation-stage laser 50 is injected in theamplification stage-laser 60, the seed light 23 is injected from onemirror (the input side mirror 1) of the mirrors forming the resonator inthe amplification stage-laser 60 while the seed light 23 is produced asamplified laser light out of another mirror (the output side mirror 2).In what follows, an account will be given of some embodiments wherein amirror for the injection of seed light 23 from the oscillation-stagelaser 50 and a mirror which the amplified seed light 23 leaves have asharing mirror function.

FIG. 25 is illustrative of one exemplary arrangement for entrance fromthe output side mirror 2 of the seed light 23 from an oscillation-stagelaser 50. FIG. 25 is illustrative in side arrangement of theoscillation-stage laser 50 and the amplification-stage laser 60. Theseed light from the line narrowing oscillation-stage laser 50 comprisinga line narrowing module 51 is reflected by two 45° right-angle prisms101 and 102 in this order, entering an exit side mirror (partialreflecting mirror) 2 that is one mirror forming a resonator in theoscillation-stage laser 60. A substantial part of the seed light 23passes through the exit side mirror 2 for injection in theamplification-stage laser 60, although the remaining slight part isreflected at the entrance surface of the exit side mirror 2. Theinjected seed light 23 passes through the discharge area 22 defined bydischarge electrodes 4 and 5 in the amplification-stage laser 60 forreflection by a rear side mirror (total-reflection mirror) 111 that isanother mirror forming the resonator in the amplification-stage laser60, whereupon the reflected seed light 23 again passes through thedischarge area 22, leaving the output side mirror 2.

In this embodiment, too, it is acceptable to inject the seed light 23having such divergence as to satisfy the above conditions (2) and (3) inthe amplification-stage laser 60. Further, if the seed light is injectedin such a way as to fill the discharge area while the optical axis C ofthe seed light 23 is slightly inclined with respect to the optical axisD of the rear- and output-side mirrors 111 and 2, much lower coherenceis then achievable so that efficient amplification and oscillation takeplace at the amplification-stage laser 60.

FIG. 26 is illustrative of the amplification-stage laser 60 in theembodiment of FIG. 25, wherein the seed light 23 is entered in thedischarge area 22 in the amplification-stage laser 60 from its end whilethe optical axis C of the seed light 23 is slightly inclined withrespect to the optical axis D of the rear side mirror 111 and the outputside mirror 2. Specifically, FIGS. 26(a) and 26(b) are a top view and aside view of the resonator in the amplification-stage laser 60,respectively.

As shown in the top view of FIG. 26(a), the narrow-banded seed light 23produced out of the oscillation-stage laser 50 is reflected by two 45°right-angle prisms 101 and 102 in this order (see FIG. 25), entering theexit side mirror 2 that is one mirror that forms the resonator in theamplification-stage laser 60. A substantial part of the seed light 23transmits through the exit side mirror 2, although the remaining slightpart is reflected at the entrance surface of the exit side mirror 2 (asindicated by a broken line). The transmitted seed light 23 is injectedfrom the side of a discharge area 22 in the amplification-stage laser60.

Entering the discharge area 22 while the optical axis C of the seedlight 23 is set at a slight angle of inclination, α, with respect to theoptical axis D of the resonator in the amplification-stage laser 60, theseed light 23 is amplified in the discharge area 22, entering a rearside mirror 111 where it is subjected to total reflection. The reflectedlight again passes through the discharge area 22 for amplification, anda part of the amplified laser light transmits through the exit sidemirror (partial reflecting mirror) 2 and is produced as laser light K1.The remaining part of the amplified laser light is reflected by the exitside mirror 2, going back to the discharge area 22 for amplification.

Then, the amplified laser light is again incident on the rear sidemirror 111 where it is subjected to total reflection. The reflectedlight again passes through the discharge area 22 for amplification, anda part of the amplified laser light transmits through the exit sidemirror (partial reflecting mirror) 2 and is produced as laser light K2.The remaining part of the amplified laser light is reflected by the exitside mirror 2, going back to the discharge area 22. By repetition ofsuch resonation, laser light K3 is produced as the output of theamplification-stage laser 60.

Here, the angle of incidence of the seed light 23 on the output sidemirror 2 and the angles of incidence and reflection of the amplifiedlight on and at the rear side mirror 111 and output side mirror 2 areeach set at an angle, α, with respect to the optical axis D of theresonator in the amplification-stage laser 60. In this way, the seedlight 23 is subjected to zigzag multiple reflections between the outputside mirror (partial reflecting mirror) 2 and the rear side mirror(total-reflection mirror) 111, as shown in the top view of FIG. 26(a).Thus, much lower spatial coherence is achievable on the same principlesas described with reference to the principles of operation of FIG. 20.Notice here that the output side mirror (partial reflecting mirror) 2has a reflectivity of, e.g., 30%. Then, the efficiency of incidence ofthe seed light 23 on the amplification-stage laser 60 will become 70%.

The advantage of this mode is that uniform coatings can be applied allover the surfaces of the rear side mirror 111 and output side mirror 2that form together the resonator in the amplification-stage laser 60; itis not necessary to apply such partial coatings as shown in FIG. 17,FIG. 18, FIG. 22, FIG. 23, and FIG. 24. This leads to another advantagethat the mirrors are easy and less expensive to fabricate, and thequality and robustness of the coatings are improved as well. Since theoutput side mirror (partial reflecting mirror) 2 has a higherreflectivity, it is acceptable to apply no coating to the injectionsite, when the efficiency of injection of the seed light 23 becomesworse.

In this embodiment, the rear side mirror 111 and the output side mirror2 are each formed of a plane mirror; however, the invention is by nomeans limited to it as long as the stable resonator is set up by bothmirrors. For instance, if the rear side mirror 111 or the output sidemirror 2 is configured as a cylindrical concave mirror, much lowerspatial coherence is then achievable. Specifically, as the cylindricalconcave mirror is located such that its generating line directionsubstantially matches the center axis of the discharge direction, itresults in a lot more resonance modes so that much lower coherence isachievable in the vertical direction to the discharge direction.

On the other hand, the energy of laser light in the resonator in theamplification-stage laser 60 will become higher than that of the laserlight produced out of the output side mirror 2 after amplification. Thiswill offer a problem in conjunction with the robustness of the rear sidemirror 111 and output side mirror 2 to laser light. However, thisproblem can be solved by timed movement of the effective portions ofthese mirrors; that robustness can be much more improved, as exemplifiedin FIG. 27.

FIG. 27 is illustrative of mirror holders for holding the rear sidemirror 111 and the output side mirror 2, respectively, as viewed fromdirections indicated by arrows E and F in FIG. 26(a). Specifically, FIG.27(a) is illustrative of a mirror holder 210 with a moving stageattached to it, as viewed from the rear side mirror 111 side (the E sideof FIG. 26(a)), and FIG. 27(b) is illustrative of a mirror holder 211with a moving stage attached to it, as viewed from the output sidemirror 2 side (the F side of FIG. 26(a)). These mirror holders 210 and211, each with the moving stage attached to it, are fixed to a plate forthe fixation of the resonator in the amplification-stage laser 60, notshown.

The mirror holder 210 for holding the rear side mirror 111 is nowexplained. The rear side mirror 111 is fixed to a mirror holder portion206, and the mirror holder portion 206 is movably fixed to a mirrorholder stage plate 203 via mirror holder guides 204 and 205. The mirrorholder portion 206 is movable by the mirror holder guides 204 and 205 inthe horizontal direction (indicated by an arrow in FIG. 27(a)) with theoptical axis remaining invariable.

One end of the mirror holder stage plate 203 on a side at a right anglewith the side provided with the mirror holder guides 204 and 205 isprovided with a screw-fixing plate 202 having a female thread portion.At this female thread portion there is held a knobbed screw 201. Theknobbed screw 201 is fixed at its distal end with a ball 212. Theknobbed screw 201 is threaded in place such that the ball 212 comes intocontact with a side portion of the mirror holder portion 206.

On the other hand, the other end portion of the mirror holder stageplate 203 on the side at a right angle with the side provided with themirror holder guides 204 and 205 is provided with a spring-fixing member208. One end of a spring 209 is fixed to the spring-fixing member 208.The other end of the spring 209 is inserted over a projection 207attached to the mirror holder portion 206. The spring 209 is designedand located such that its resilient force allows the mirror holderportion 206 to be forced against the ball 212 fixed to the distal end ofthe knobbed screw 201. Notice here that the projection 207 attached tothe mirror holder 206 is located at a position substantially coaxialwith the knobbed screw 201.

With such an arrangement, as the knobbed screw 201 is rotated, it allowsthe rear side mirror 111 to translate horizontally with its optical axisremaining invariable. The mirror holder 211 for holding the exit sidemirror 2 is constructed as in the mirror holder 210.

Preferably in this embodiment, the mirror holders 210 and 211 should besymmetric with respect to a plane vertical to the paper sheet of FIG. 27passing through an XX axis such that the knobbed screws 201 of themirror holders 210 and 211 are positioned on the same side as theamplification-stage laser 60 and their maintenance sides are positionedin the same direction.

In the embodiment of FIG. 27, if the rear side mirror 111 and the outputside mirror 2 are moved using the mirror holders 210 and 211, then thesame mirrors 111 and 2 are each used three times with the output laserlight 213, so that the service life of each mirror can be extended threetimes. In this embodiment, the mirror holder portion 206 is operable tomove in one direction alone. However, the invention is not limited toit; for instance, the mirror holder portion 206 could be located on atwo-axis stage. In FIG. 27, there is not shown the mirrorinclination-adjustment mechanism necessary for the adjustment of theoptical axis of the rear side mirror 111 and the output side mirror 2;however, that mechanism could be located on the mirror holder 206.

Another embodiment of the arrangement wherein the mirror for theinjection of the seed light 23 from the oscillation-stage laser 50 andthe mirror for producing the amplified laser light out of the seed light23 have a sharing mirror function is now explained with reference toFIG. 28.

The amplification-stage laser 60 shown in FIG. 28 is used as theamplification-stage laser 60 in the two-stage laser system shown in FIG.25 in place of the amplification-stage laser 60 shown in FIG. 26. Astructural difference with the amplification-stage laser 60 shown inFIG. 26 is that an optical element for turning light back using atotal-reflection right-angle prism (roof prism) 103 is used in lieu ofthe rear side mirror (total-reflection) mirror 11 in the resonatorlocated in the amplification-stage laser 60 of FIG. 26. Other componentsare the same as in FIG. 26.

That is, FIG. 28 shows the amplification-stage laser 60 wherein theoptical axis C of the seed light 23 is slightly inclined with respect tothe optical axis D of the total-reflection right-angle prism 103 and theoutput side mirror 2 to enter the seed light 23 in the discharge area 22in the amplification-stage laser 60 from its end. Specifically, FIGS.28(a) and 28(b) are a top view and a side view of the resonator in theamplification-stage laser 60, respectively.

In this embodiment, too, the optical axis C of the seed light 23 isslightly inclined with respect to the optical axis D of thetotal-reflection right-angle prism 103 and the output side mirror 2 toinject the seed light 23 in such a way as to fill the discharge area, aspreviously described. Therefore, much lower coherence is achievable, andso efficient amplification and oscillation are achievable at theamplification-stage laser 60.

As shown in the top view of FIG. 28(a), the narrow-banded seed light 23produced out of the oscillation-stage laser 50 is reflected by two 45°right-angle prisms 101 and 102 in this order (see FIG. 25), entering theoutput side mirror (partial reflecting mirror) 2 that is one opticalelement forming the resonator in the amplification-stage laser 60. Asubstantial part of the reflected light transmits through the exit sidemirror 2, although the remaining slight part is reflected at theentrance surface of the exit side mirror 2 (indicated by a broken linein FIG. 28(a). The transmitted seed light 23 is injected from the sideof the discharge area 22 in the amplification-stage laser 60.

This seed light 23 enters the amplification-stage laser 60 with itsoptical axis C set at a slight angle of inclination, α, with respect tothe optical axis D of the resonator in the amplification-stage laser 60.In the discharge area 22, the seed light 23 is amplified, and thensubjected to Fresnel total reflection at the surfaces 103 ₁ and 103 ₂ ofthe total-reflection right-angle prism 103 (reflection at an angle ofincidence larger than the critical angle).

Notice here that this embodiment works differently than the embodimentof FIG. 26. More specifically, the incident laser light is totallyreflected twice at the surfaces 103 ₁ and 103 ₂ of the total-reflectionright-angle prism 103, so that the output laser light goes back the waythat it has come. This turned-back laser light again passes through thedischarge area 22 where it is amplified. A part of the amplified lighttransmits through the exit side mirror (partial reflecting mirror) 2,and is produced as laser mirror K1. The rest is reflected by the exitside mirror 2, going back to the discharge area 22 where it isamplified.

Then, the amplified laser light again enters the total-reflectionright-angle prism 103 where it is totally reflected. The totallyreflected light again goes back the way that it has come, again passingthrough the discharge area 22 where it is amplified. A part of theamplified laser light transmits through the exit side mirror (partialreflecting mirror) 2, and is produced as laser light K2. The rest isreflected by the exit side mirror 2, going back to the discharge area 22where it is amplified. By repetition of such resonance, laser light K3is produced as the output of the amplification-stage laser 60.

Here the angle of incidence of the seed light 23 on the output sidemirror 2 and the angles of incidence and reflection of the amplifiedlight on and at the total-reflection right-angle prism 103 and theoutput side mirror 2 becomes α. In this way, as shown in the top view ofFIG. 28(a), the seed light 23 is subjected to zigzag multi-reflectionbetween the output side mirror (partial reflecting mirror) 2 and thetotal-reflection right-angle prism 103. Thus, lower spatial coherence isachievable on the same principles as described with reference to FIG. 23that is illustrative of the principles of operation.

With the embodiment explained with reference to FIG. 28, the additionalfollowing advantages are obtainable in addition to the advantagesequivalent to those previously explained with reference to FIG. 26. Inthis embodiment, the laser light is turned back by the total-reflectionright-angle prism 103. For this reason, even when there is an unevenamplification intensity distribution in the longitudinal direction(discharge direction) of the amplification gain that is the dischargearea 22, the symmetry and stability of the output laser light in thelongitudinal direction are improved because the laser light passesthrough both upper and lower regions of the discharge area 22.

More specifically, as the frequency of repetition of laser oscillationgrows (e.g., 3,000 to 4,000 Hz), discharge between the dischargeelectrodes 4 and 5 causes standing waves to occur due to acoustic waves,giving rise to uneven amplification gain distribution and refractiveindex in the longitudinal direction (discharge direction). On thecontrary, if the laser light is turned back by the total-reflectionright-angle prism 103 for re-amplification, then it is possible tomaintain the post-amplification uniformity, symmetry and stability ofthe laser light. It is also possible to achieve much lower coherence.Notice here that to obtain such advantages, the ridgeline of thereflecting surfaces 103 ₁ and 103 ₂ of the total-reflection right-angleprism 103 (the ridgeline of the roof) must be directed in asubstantially vertical direction to the discharge direction (see FIG.28(b)).

In this embodiment, the entrance surface of the total-reflectionright-angle prism 103 may or may not be applied with an antireflectioncoating. However, the entrance surface of the total-reflectionright-angle prism 103 must be inclined with respect to the reflectingsurface of the output side mirror 2 for the purpose of preventingparasitic oscillation with respect to the amplification resonator andthe optical axis C of the seed light.

In the above embodiment of the two-stage laser system, when the seedlight 23 is injected from the oscillation-stage laser 50 in theamplification-stage laser 60, the seed light 23 is injected therein fromone mirror (input side mirror 1) forming the resonator therein, and theseed light 23 is produced as amplified laser light out of the othermirror (output side mirror 2). The mirror for the injection of the seedlight 23 from the oscillation-stage laser 50 and the mirror out of whichthe seed light 23 is produced as amplified laser light have a sharingfunction. In any case, the seed light 23 passes through the dischargearea 22 just upon entrance and transmission of the seed light 23 in andthrough one mirror that forms the resonator in the amplification-stagelaser 60.

FIG. 29 and FIG. 30 show a modification to the embodiment of theinvention wherein the seed light 23 is injected in theamplification-stage laser 60 from one mirror (input side mirror 1) thatforms the resonator therein and produced as amplified laser light out ofthe other mirror (output side mirror 2). More specifically, the seedlight 23 enters and transmits through the input side mirror 1 in theamplification-stage laser 60, and arrives at the output side mirror 2through an area other than the discharge area 22, where it is reflected.Then, the reflected light passes through the discharge area 22.

If viewed from the discharge area 22, this embodiment will be equivalentto the case where the mirror for the injection of the seed light 23 fromthe oscillation-stage laser 50 and the mirror out of which the seedlight 23 is produced as amplified laser light have a sharing function.That is, the seed light 23 enters the discharge area 22 from its outputside, where it is amplified, and then leaves the discharge area 22.

FIG. 29 is illustrative of one exemplary embodiment of theamplification-stage laser 60. Specifically, FIGS. 29(a) and 29(b) are atop view and a side view of the resonator in the amplification-stagelaser 60.

As shown in the view of FIG. 30(a) as taken from a direction indicatedby an arrow E in FIG. 29(a), the construction of the input side mirror 1in this embodiment is the same as that of the input side mirror 1 shownin FIG. 22, FIG. 23 and FIG. 24. That is, an antireflection coating 9 isapplied to the area of a CaF₂ or other plane substrate transparent toultraviolet light, which is to receive the seed light 23, and ahigh-reflectivity (total-reflection) coating 8 is applied to the rest.

On the other hand, the construction of the output side mirror 2 is shownin the view of FIG. 30(b) as taken from a direction indicated by anarrow F in FIG. 29(a). A high-reflectivity (total-reflection) mirrorcoating 8 is applied to the area of a CaF₂ or other plane substratetransparent to ultraviolet light, which is to receive the seed light 23,and a partial reflecting mirror coating 10 is applied to the rest.

Referring to FIG. 29, the seed light 23 produced out of theoscillation-stage laser 50 (see FIG. 19 or FIG. 21) enters and transmitsthrough a transmitting portion of the input side mirror 1 (the areaprovided with the anti-reflection coating 9) and then through an area ofthe amplification-stage laser 60 other than the discharge area 22, andenters a total-reflection portion of the output side mirror 2 (the areaapplied with the high-reflectivity (total-reflection) mirror coating 8),at which it is totally reflected toward the discharge area 22.

In the amplification-stage laser 60 shown in FIG. 29, the optical axis Cof the seed light 23 is inclined by a slight angle α with respect to theoptical axis D of the resonator.

In this embodiment, too, the optical axis C of the seed light 23 isslightly inclined with respect to the optical axis D of the resonator tofill the discharge area with the seed light by injection. Therefore,much lower coherence is achievable, and efficient amplification andoscillation take place at the amplification-stage laser 60 as well, aspreviously described.

Upon incidence on the total-reflection portion of the output side mirror2 and total reflection toward the discharge area 22, the seed light 23passes through the discharge area 22 where it is amplified. Then, theamplified laser light enters the total-reflection portion of the inputside mirror 1 (the area applied with the high-reflectivity(total-reflection) mirror coating 8), where it is totally reflected.

The reflected light again passes through the discharge area 22 where itis amplified, entering the partial reflecting portion of the output sidemirror 2 (the area applied with the partial reflecting mirror coating10), where it is amplified. A part of the amplified laser lighttransmits through the output side mirror 2, leaving it as laser lightK1. The rest is reflected there, going back to the discharge area 22.

The reflected light that has gone back to the discharge area 22 againpasses through the discharge area 22 where it is amplified. Then, theamplified light enters the total-reflection portion of the input sidemirror 1, where it is totally reflected. Then, the amplified laser lightenters the partial reflecting portion of the output side mirror 2, whereit is amplified. A part of the amplified laser light transmits throughthe output side mirror 2, leaving it as laser light K2. The rest isreflected there, going back to the discharge area 22. By repetition ofsuch resonance, T3 is produced as the output of the amplification-stagelaser 60.

Here the angle of incidence of the seed light 23 on the output sidemirror 2 and the angles of incidence and reflection of the amplifiedlight on and at the input side mirror 1 and the output side mirror 2become a with respect to the optical axis D of the resonator in theamplification-stage laser 60.

In this way, the seed light 23 is subjected to zigzag multiplereflections between the output side mirror 2 and the input side mirror1, as shown in the top view of FIG. 29(a).

The advantage of this embodiment is that the seed light 23 can beinjected with efficiency in the amplification-stage laser. FIG. 30(a) isillustrative of in what relation the input side mirror 1 is positionedto the discharge area and the seed light 23 as viewed from a directionindicated by an arrow E in FIG. 29(a), and FIG. 30(b) is illustrative ofin what relation the output side mirror 2 is positioned to the dischargearea and the seed light 23 as viewed from a direction indicated by anarrow F in FIG. 29(a).

In this embodiment, the seed light 23 is entered from a transmittingportion of the input side mirror 1 in the discharge area 22 in theamplification-stage laser 60 at a position slightly spaced away from it.As viewed from a direction E (see FIG. 29(a)), the discharge area 22 ispositioned such that it overlaps the total-reflection portion of theinput side mirror 1 and the end of the discharge area 22 substantiallymatches the end of the total-reflection portion (FIG. 30(a)). As viewedfrom a direction F (see FIG. 29(a)), the discharge area 22 is positionedsuch that the boundary line between the total-reflection portion and thepartial reflection portion of the output side mirror 1 substantiallymatches the end of the discharge area 22 and the discharge area 22overlaps the partial reflection portion (FIG. 30(b)).

No application of the coating 10 to the transmitting portion of theinput side mirror 1 and the partial reflecting portion of the outputside mirror 1 provides three advantages as described just below.

(1) A common material can be used for both mirrors 1 and 2.

(2) Both mirrors 1 and 2 are easier to fabricate.

(3) There is no partial reflecting film (coating 10) at the partialreflecting portion of the output side mirror 2; robustness is improvedbecause of no coating deterioration (even when there is no partialreflecting mirror coating 10, Fresnel reflection allows the output sidemirror to work as a partial reflecting mirror).

When, as in the embodiment of FIG. 20 or FIG. 29, the seed light 23 isinjected from the input side mirror 1 in the amplification-stage laserwith its optical axis C slightly inclined with respect to the opticalaxis D of the resonator, too, it is acceptable to use thetotal-reflection right-angle prism (roof prism) employed in theembodiment of FIG. 28 in place of the input side mirror 1 applied withthe high-reflectivity (total-reflection) mirror coating 8.

By the way, the inventors have found that when the seed light 23 isentered in the discharge area 22 in the amplification-stage laser 60from its end while, as in FIG. 20, FIG. 26 and FIG. 29, the optical axisC of the seed light 23 is slightly inclined with respect to the opticalaxis D of the plane input side mirror 1 or the rear side mirror 111 andthe plane output side mirror that form the resonator in theamplification-stage laser 60, it is possible to run the laser systemmore efficiently while keeping low-coherent characteristics, forinstance, by properly inclining the input side mirror 1 or the rear sidemirror 111 with respect to the output side mirror 2.

First of all, consider the degree of flexibility in the condition forthe injection of seed light 23. Typically in the arrangement of FIG. 31analogous to that of FIG. 19, the length, L, of the resonator is definedby the distance between the input side mirror (or the rear side mirror)1 and the output side mirror 2 in the amplification-stage laser 60, andthe effective width of the discharge area 22 effective for amplificationis defined as Wx (mm) in the section of FIG. 19(a), and Wy (mm) in thesection of FIG. 19(b).

In view of the position and angle of the seed light 23 at the positionof the input side mirror 1, consider the condition under which the seedlight 23 can make a given frequency of roundtrips in the resonator(input side mirror 1 and output side mirror 2) in theamplification-stage laser 60 to effectively obtain it as laser output.For instance, when the seed light 23 enters a portion of the input sidemirror (rear side mirror) 1 applied with a high-reflective coating, asshown in the schematic section of FIG. 32 as taken in the verticaldirection (x direction) to the resonator in the amplification-stagelaser 60 and the discharge direction of the discharge area 22, it willbe reflected by the input side mirror (rear side mirror) 1 at whateverangle. Therefore, the seed light will be incapable of being effectivelytaken as laser output.

Next, consider the incidence of the seed light 23 from a position nearthe edge portion of the input side mirror (rear side mirror) 1. As theangle of incidence is too shallow (or as the seed light 23 is incidentat an angle almost vertical to the output side mirror 2), the seed light23 will be incapable of entering the discharge area 22, and it will beincapable of entering the high-reflectivity mirror coating area of theinput side mirror (rear side mirror) 1 upon making roundtrips in theresonator; in any case, the seed light 23 will escape from the system.

As the angle of incidence is too tight (or the seed light 23 isobliquely incident on the output side mirror 2), conversely, the seedlight 23 will deviate from the discharge area 22 after reflection at theoutput side mirror 2; it will not provide any effective laser output.

From such points of view, it is possible to derive the conditionnecessary for the position and angle of the seed light 23 at the inputside mirror (rear side mirror) 1, under which the seed light 23 makes agiven frequency of roundtrips in the resonator in theamplification-stage laser 60 to provide effective output laser light.

As shown in FIG. 35 as an example, assume that a z-axis is set in adirection that passes the center of the discharge area 22 and runs alongthe discharge electrodes; an x-axis is set in a direction vertical tothe discharge direction, the origin is set at the position of the inputside mirror (rear side mirror) 1; with respect to the x-axis, an uppersite of the paper is taken as positive; and with respect to the anglesof the seed light 23 and the mirror 1, 2, the counterclockwise directionis taken as positive, and when the normal axis to the mirror lies in az-axis direction, those angles are taken as zero. Here let Xin and minbe the position and angle of injection of the seed light 23 at theposition of the input side mirror (rear side mirror) 1, L be theresonator length, θ′ be the angle of inclination of the input sidemirror (rear side mirror) 1, and the angle of inclination of the outputside mirror 2 be zero. Then, the positions Xn and Xn+0.5 of the seedlight 23 at an n-th roundtrip (the position of the input side mirror(rear side mirror) 1) or a n+0.5-th roundtrip (the position of theoutput side mirror 2) are written asXn=Xin+2n·L·θin+2n(n−1)L·θ′  (5)Xn+0.5=Xin+(2n+1)·L·θin+2n ² ·L·θ′  (6)

From these equations, the position and angle of injection of the seedlight 23 at the position of the input side mirror (rear side mirror) 1needed to take effective laser output out of the output side mirror 2with no deviation from the discharge area 22 are calculated depending onthe frequency of roundtrips to be taken into account.

Typically, consider now the embodiment (FIG. 31) wherein the input sidemirror 1 is decentered in the horizontal direction (x-axis direction),and assume that the seed light makes six roundtrips under the conditionsthat the resonator length is L=1 m, the discharge width is Wx=2.5 mm,the input side mirror 1 is arranged parallel with the output side mirror2, and the edge of the input side mirror 1 is in alignment with the endof the discharge area 22 (FIG. 36). As long as the polygonal regioncondition shown in FIG. 27 is satisfied, it is possible to obtaineffective laser output. Although depending on the size, beam divergenceangle, etc. of the seed light 23 to be injected, it is desired that thearea of this polygonal region be as large as possible.

In FIG. 37, the input side mirror 1 is arranged parallel with the outputside mirror 2. When the input side mirror 1 is inclined by +0.04 mrad,on the other hand, a region capable of effectively taking laser lightmay be found from FIG. 38. As can be seen from FIG. 38, slightinclination of the input side mirror 1 ensures that there is an increasein the area of the region capable of effectively taking laser light(which results in an increase in the degree of flexibility in theinjection of the seed light 23), and low coherence is achievable as inthe case of a parallel arrangement. It is thus possible to achieve alaser system having improved output.

In this case, the input side mirror 1 is inclined with respect to theoutput side mirror 2 in such a direction that in view of the distance Lbetween the input side mirror 1 and the output side mirror 2, the seedlight 23 oscillated out of the oscillation-stage laser 50 is incidentfrom the side where the mirror-to-mirror spacing becomes wide with theinclination of one mirror.

With this arrangement wherein the resonator built up of two planemirrors in the amplification-stage laser 60 is set such that one mirroris slightly inclined, not in parallel, with respect to the other, thewidth of spectra occurring through discharge at the amplification-stagelaser 60 decreases in gain relative to broad natural light emissions,with the result that the broadband ratio becomes lower than that in thearrangement wherein two mirrors are arranged in parallel. In otherwords, it is required for the oscillation-stage laser 50 to have thedesired peak intensity so as to meet the requirement for the desiredbroadband ratio or lower, as set forth in Japanese Patent ApplicationNo. 2003-130447. As described above, however, if the resonator is builtup of two non-parallel mirrors, then the peak level can be much morereduced down.

In view of the frequency of roundtrips in the resonator, there is alarge difference between when the resonator mirrors are parallel andwhen they have a mutual proper inclination, which ensures that there isan extension of the pulse width of laser light. In consideration of theservice life of a semiconductor aligner, it is desired that the laserpulse width be as long as possible.

This is now considered in detail. When the resonator mirrors have amutual proper inclination as shown in FIG. 39(a), the seed light 23 isobliquely incident on the resonator due to the relation of the edgeportion of the input side mirror 1 to the discharge area 22. When onemirror is properly inclined as shown in FIG. 39(a), the seed light canmake more roundtrips in the resonator in the amplification-stage laseras compared to when the resonator mirrors are parallel as shown in FIG.39(b), so that far higher laser output is achievable with a furtherextension of the pulse width.

It is noted that when the inclination of the input side mirror 1 rangesfrom 0.0 mrad to 0.16 mrad, the ensuing laser system output surpassesthat of the laser arrangement wherein the high-reflectivity side planeof the input side mirror 1 is parallel with the partial-reflectivityside plane of the output side mirror 2. It is to be understood that thisrange also varies with changes in the resonator length, discharge widthand the frequency of roundtrips to be taken into account, as can be seenfrom the aforesaid equations (5) and (6). For instance, given threeroundtrips, output surpassing that of the parallel resonator mirrorarrangement will be obtained in the range of 0.0 mrad to 0.87 mrad.

In any case, the above range can relatively easily be derived on thebasis of equations (5) and (6), and if the resonator is designed whilethis range is taken into consideration, increased laser output, extendedpulse width, the degree of flexibility in the injection of the seedlight 23 and decreased peak intensity of the oscillation-stage laserwill then be achieved.

From another point of view, consider here how the input side mirror(rear side mirror) 1 and the output side mirror 2 are inclined withrespect to the optical axis C of the seed light 23 (see FIG. 20) in thecase where two plane mirrors forming the resonator in theamplification-stage laser 60 are located in a non-parallel fashion.

Unlike the case of FIG. 35, set a z-axis in the direction of travel ofthe seed light 23 and an x-axis in the discharge direction or in thevertical direction to the discharge direction with the origin at aposition of the input side mirror (rear side mirror) 1, on which theseed light 23 is to be incident, as shown in FIG. 40, and assume thatwith respect to the x-axis, an upper site of the paper is taken aspositive; with respect to the angles of the seed light 23 and the mirror1, 2, the clockwise direction is taken as positive; and when the normalto the mirror 1, 2 lies in the z-axis direction, those angles are takenas zero. Here let OR be the angle of inclination of the input sidemirror (rear side mirror) 1 and OF be the angle of inclination of theoutput side mirror 2. The seed light 23 is assumed to be injected in theorigin of the x-axis coordinates at an angle of inclination of zero.

The angle at which the light travels after reflection at each mirror 1,2 is written asθ_(NF)=2Nθ _(F)−2(N−1)θ_(R)θ_(NR)=2Nθ _(F)−2Nθ _(R)Here the suffixes “NF” and “NR” represent light rays after an N-threflection at the output side mirror 2 and the input side mirror (rearside mirror) 1, respectively.

The coordinates for the point of reflection after N roundtrips arewritten asX _(NR)=2N ²θ_(F) L−2N(N−1)θ_(R) LX _(NF)=2N(N+1)θ_(F) L−2N ²θ_(R) LHere the suffixes “NF” and “NR” represent the points of N-th reflectionat the output side mirror 2 and the input side mirror (rear side mirror)1, respectively, and L stands for the length of the resonator in theamplification-stage laser.

Unless X_(1R)>0, the light will not be reflected at the input sidemirror (rear side mirror) 1 after one roundtrip. It is thereforerequired to satisfyθ_(F)>0

Upon N roundtrips, the condition for reflecting light at the input sidemirror (rear side mirror) 1 becomesX_(NR)>0Therefore,θ_(R) <N/(N−1)×θ_(F)

From the foregoing, the conditions for reflecting light at the inputside mirror (rear side mirror) 1 upon N roundtrips becomeθ_(F)>0, andθ_(R) <N/(N−1)×θ_(F)

Now, to make more resonances in the effective amplification area ascompared with (θ_(F)=θ_(R)), position variations in the input sidemirror (rear side mirror) 1 or the output side mirror 2 must be reducedwith the frequency of roundtrips in the resonator. Similar results areobtained with any mirror; reference is then made to the output sidemirror 2.

The above condition becomesX _(N+2 F) −X _(N+1 F) <X _(N+1 F) −X _(NF)Therefore,θ_(F)<θ_(R)

From a combination of this with the conditions as provided above, thecondition for reflecting light at the output side mirror 1 up to Nroundtrips and reducing the position variations becomes0<θ_(F)<θ_(R) <N/(N−1)×θ_(F)  (7)

That is, it is required in FIG. 40 that both the input side mirror (rearside mirror) 1 and the output side mirror 2 be inclined in the clockwisedirection, and when the angle of inclination of the output side mirror 2is θ_(F), the angle of inclination θ_(R) of the input side mirror (rearside mirror) 1 be somewhat lager than θ_(F). This is tantamount to theinjection of the seed light 23 from the side where the distance betweenthe mirrors 1 and 2 is longer and they are mutually open.

Typically, given N=5 and θ_(F)=0.5 mrad,0.5 mrad<θ_(R)<0.625 mradso that the angle of aperture between the input side mirror (rear sidemirror) 1 and the output side mirror 2 lies in the range of 0 to 0.125mrad.

In the above discussions, absolute values are not attached to theinequality regarding position variations. However, it is more desirousto attach the absolute value to the inequality for comparison purposes.The reason could be that when there is no absolute value sign, anyshifts of light in the negative direction of the x-axis are allowable.

In the state with the absolute value signs attached, from|X_(N+2 F)−X_(N+1 F)|<|X_(N+1 F)−X_(NF)|,θ_(F)<θ_(R)<(2N+3)/(2N+2)×θ_(F)is derived. From a combination with the condition of θ_(F)>0, thecondition for reflecting light at the output side mirror 2 up to Nroundtrips and reducing the absolute value of position variationsbecomes0<θ_(F)<θ_(R)<(2N+3)/(2N+2)×θ_(F)  (8)

Typically, given N=5 and θ_(F)=0.5 mrad,0.5 mrad<θ_(R)<0.542 mradThat is, the angle of aperture between the input side mirror (rear sidemirror) 1 and the output side mirror 2 falls in the range of 0 to 0.042mrad.

In any event, it is found that both the input side mirror (rear sidemirror) 1 and the output side mirror 2 must be inclined in the samedirection with respect to the optical axis C of the seed light 23, theangle of inclination θ_(R) of the input side mirror (rear side mirror) 1must be somewhat larger than the angle of inclination θ_(F) of theoutput side mirror 2, and the seed light 23 must be injected from theside on which the distance between the mirrors 1 and 2 is longer and theangle of aperture between them is larger. It is then preferable that theangle of aperture between both mirrors 1 and 2 is in the range of 0.01mrad to 0.2 mrad. It is here noted that when the seed light 23 isinjected from the output side mirror 2 side, θ_(R) and θ_(F) areinterchangeable.

FIG. 31 is illustrative of an embodiment of the invention wherein, as inFIG. 16, both the input side mirror 1 and the output side mirror 2 areinclined in the same direction with respect to the optical axis C of theseed light 23, the angle of inclination of the input side mirror 1 issomewhat larger than the angle of inclination of the output side mirror2, and the seed light 23 is injected from the side on which the distancebetween both the mirrors 1 and 2 is longer and the angle of aperturebetween them is larger. For the purpose of illustration, the seed light23 makes a mere 1.5 roundtrips in the amplification-stage laser 60; inactual applications, however, there are set a lot more roundtrips. Forthe purpose of illustration, light is drawn as one single line; inactual applications, however, the light is a beam having a finite widthand a finite divergence angle. Throughout the following drawingsregarding the inclinations of the input side mirror (rear side mirror111) 1 and the output side mirror 2, mirrors and the degree ofinclination of optical axies are exaggerated for convenience ofillustration.

As in FIG. 19, this is directed to an embodiment wherein the input sidemirror 1 in the amplification-stage laser 60 is formed of onehigh-reflectivity (total-reflection), hole-free plane mirror. Typically,this input side mirror 1 is applied with a high-reflectivity coating onthe entire surface of its side near to the chamber 3 in theamplification-stage laser 60. The input side mirror 1 is also typicallyprovided on its entire back surface with a reflectivity-free coating(anti-reflection coating) and/or a suitable wedge angle for the purposeof preventing interferences between the two surfaces. The output sidemirror 2 is typically provided on the entire surface of its side near tothe chamber 3 in the amplification-stage laser 60 with a partialreflecting mirror coating (having a reflectivity of typically 10% to50%) in such a way as to have an optimum reflectivity in the lasersystem. The output side mirror 2 is also typically provided on itsentire back surface with a reflectivity-free coating (antireflectioncoating) and/or a suitable wedge angle for the purpose of preventinginterferences between the two surfaces.

The input side mirror 1 is located such that it is decentered in thehorizontal direction (within the plane of the top view (a) paper) withrespect to the seed light 23 from the oscillation-stage laser 50, andits high-reflectivity side plane is not parallel with the partialreflectivity side plane of the output side mirror 2. More specificallyin view of the top view (a), that plane is located in such a way as tohave a proper inclination and the edge of the input side mirror 1 ispositioned within or near the discharge area 22 defined by the dischargeelectrodes 4 and 5 in the amplification-stage laser 60. In view of thehigh-reflectivity side plane of the input side mirror 1 and the partialreflectivity side plane of the output side mirror 2, the direction ofthat inclination is such that at the edge portion of the input sidemirror 1 in which the seed light 23 is to be introduced, the distancebetween the two mirrors is longer than that between the oppositemirrors. Then, to satisfy inequality (7) or (8) as described above, theangle of inclination of the input side mirror 1 with respect to theoptical axis C of the seed light 23 is somewhat larger than that of theoutput side mirror 2 on the same side.

With this arrangement, it is possible to prevent pits from occurring inthe profile of the laser light produced out of the amplification-stagelaser 60 (spots of weak light intensity in the center beam portion).

The value of the “proper inclination β” used herein has previously beenspecified. More specifically in a laser system having fixed otherfactors such as gas pressure, applied voltage, and energy of the seedlight 23, that value is set such that the laser system output lies inthe range G that does not fall short of the output S of the laser systemwherein the high-reflectivity side plane of the input side mirror 1 isparallel with the partial reflectivity side plane of the output sidemirror 2 (FIG. 20).

FIG. 42 is illustrative of one exemplary arrangement wherein theresonator in the amplification-stage laser 60 is made up of twonon-parallel mirrors, and the seed light 23 from the oscillation-stagelaser 50 is entered from the output side mirror 2, as in FIG. 25. FIGS.42(a) and 42(b) are a top view and a side view of that arrangement,respectively, and FIG. 42(c) is illustrative of the output side mirror 2in the amplification-stage laser 60, as viewed from the chamber 3 side.In this arrangement, the seed light 23 is introduced from outside theedge of the output side mirror 2 along it. In this case, the seed light23 is entered from the output side mirror 2, and so the opposite mirroris called the rear side mirror 111. Typically, the rear side mirror 111is applied with a high-reflectivity coating all over the surface of theside near the chamber 3 in the amplification-stage laser 60. The entireback surface of the rear side mirror 111 is typically applied with areflectivity-free coating (anti-reflection coating) and/or a suitablewedge angle for the purpose of preventing interferences between the twosurfaces. The output side mirror 2 is typically provided on the entiresurface of its side near the chamber 3 in the amplification-stage laser60 with a partial reflecting mirror coating (having a reflectivity oftypically 10% to 50%) in such a way as to have an optimum reflectivityin the laser system. The output side mirror 2 is typically provided onits entire back surface with a reflectivity-free coating (antireflectioncoating) and/or a suitable wedge angle for the purpose of preventinginterferences between the two surfaces.

In this case, the output side mirror 2 is located such that it isdecentered in the horizontal direction (within the plane of the top view(a) paper) with respect to the seed light 23 from the oscillation-stagelaser 50, and its partial reflectivity side plane is not parallel withthe high-reflectivity side plane of the rear side mirror 111. Morespecifically in view of the top view (a), that plane is located in sucha way as to have a proper inclination and the edge of the output sidemirror 2 is positioned within or near the discharge area 22 defined bythe discharge electrodes 4 and 5 in the amplification-stage laser 60. Inview of the partial reflectivity side plane of the output side mirror 2and the high-reflectivity side plane of the rear side mirror 111, thedirection of that inclination is such that at the edge portion of theoutput side mirror 2 into which the seed light 23 is to be introduced,the distance between the two mirrors is longer than that between theopposite mirrors. In this case, the angle of inclination of the outputside mirror 2 with respect to the optical axis C of the seed light 23 issomewhat larger than that of the rear side mirror 111 on the same side(contrary to FIG. 40).

The value of the “proper inclination β” used herein, too, has previouslybeen specified. More specifically, as shown in FIG. 41, that value isset such that the laser system output lies in the range G that does notfall short of the output S of the laser system wherein thehigh-reflectivity side plane of the rear side mirror 111 is parallelwith the partial reflectivity side plane of the output side mirror 2(FIG. 20). In view of the partial reflectivity side plane of the outputside mirror 2 and the high-reflectivity side plane of the rear sidemirror 111, the direction of that inclination is such that at the edgeportion of the output side mirror 2 into which the seed light 23 is tobe introduced, the distance between the two mirrors is longer than thatbetween the opposite mirrors.

One advantage of this arrangement is that smaller seed light can be usedas the seed light 23, because upon injection in the amplification-stagelaser 60, it is the rear side mirror 111 of high reflectivity that itstrikes at first. As shown in FIG. 42(c), however, a problem with thearrangement is that the output side mirror 2 is decentered for theentrance of the seed light 23, and so the beam is of somewhat limitedsize. In FIG. 42, the seed light 23 is shown to make only two roundtripsin the amplification-stage laser 60 for the purpose of illustration; inactual applications, however, there are set a lot more roundtrips.Likewise for the purpose of illustration, the light is drawn as onesingle line; however, it is a beam having a finite width and a finitedivergence angle.

In the embodiment of FIG. 31, the input side mirror 1 is made up of onehigh-reflectivity (total-reflection), hole-free plane mirror. As shownin FIG. 43(a) as viewed from the chamber 3 side and in the sectionalview of FIG. 43(b), however, the input side mirror 1 could be achievedby applying the antireflection coating 9 to a seed light-incident areaof the output side of a CaF₂ or other plane substrate transparent toultraviolet light and the high-reflectivity (total-reflection) mirrorcoating 8 to the rest. That is, with the input side mirror 1 used inFIG. 31, there is difficulty in the application of coating as far as themirror end faces for the purpose of holding the mirror during vapordeposition. In addition, it is not easy to process the ends of the CaF₂or other substrate into right-angle faces with high precision; usually,there is a chip off the ends during fabrication. Without chip-freeapplication of the high-reflectivity (total-reflection) coating 8 as faras the ends, the substrate ends having a decreased reflectivity willcause losses leading to an oscillation efficiency drop. If such an inputside mirror 1 as shown in FIG. 43 is used, it will be easy to processthe ends of the high-reflectivity (total-reflection) coating 8; it willbe possible to apply the high-reflectivity coating 8 as far as theboundary between the seed light 23 and the amplified laser light in theamplification-stage laser 60. Alternatively, the antireflection coating9 could be dispensed with; there could be no coating at all. FIG. 44 isillustrative of an embodiment corresponding to FIG. 31, wherein thisinput side mirror 1 is used.

In the embodiment of FIG. 42, the output side mirror 2 is made up of onepartial reflectivity, hole-free plane mirror. As shown in FIG. 45(a) asviewed from the chamber 3 side and in the sectional view of FIG. 44(b),however, the output side mirror 2 could be achieved by applying theantireflection coating 9 to a seed light-incident area of the chamber 3side of a CaF₂ or other plane substrate transparent to ultraviolet lightand the partial reflecting mirror coating 10 to the rest for similarreasons. FIG. 46 is illustrative of an embodiment corresponding to FIG.42, wherein this output mirror 2 is used.

While, in the above embodiments, the input side mirror 1, the rear sidemirror 111 and the output side mirror 2 are all in rectangular form, itis to be understood that they could have any desired shape withoutdeviating from the purport of the invention.

Further, when the resonator in the amplification-stage laser 60 is builtup of two nonparallel mirrors, the input side mirror 1 could be locatedwhile decentered in the vertical direction with respect to the seedlight 23, as in FIG. 21. As long as at the edge portion of the inputside mirror 1 in which the seed light 23 is to be introduced, thedistance between the two mirror portions is longer than that between theopposite mirror portions, the mirrors 1 and 2 could have any desiredinclination sections. FIG. 47 is illustrative of an embodiment whereinthe resonator in the amplification-stage laser 60 is built up of twononparallel mirrors, as in FIG. 21, and FIG. 47(b) is a side view ofthat embodiment wherein the input side mirror 1 and the output sidemirror 2 are inclined with respect to the optical axis C of the seedlight 23. In this case, there is shading upon injection of the seedlight 23, and so the laser system efficiency becomes somewhat lower thancould be achieved with the above embodiments described with reference tothe top views.

In this case, too, it is to be understood that the mirrors 1 and 2 areset at such an angle of inclination that the laser system output lies inthe range G that does not fall short of the output S of the laser systemwherein the high-reflectivity side plane of the input side mirror 1 isparallel with the partial reflectivity side plane of the output sidemirror 2 (FIG. 20). Alternatively, the injection area could be ensuredby coating rather than at the edges of the mirrors 1 and 2 (see FIGS.43-46).

Throughout all the two-stage laser systems for aligners of the inventiondescribed above, the seed light 23 emitted out of the oscillation-stagelaser 50 is introduced in the resonator in the amplification-stage laser60 from the side of the input side mirror 1 or the output side mirror 2that form that resonator. It is understood, however, that the seed light23 could be introduced in the direction of the laser oscillation opticalaxis of the amplification-stage laser 60 from any desired positionbetween the resonator mirrors 1 and 2. In such a case, the mirror thatopposes the output side mirror 2 will in no sense be any input sidemirror. Therefore, that mirror will hereinafter be called the rear sidemirror 111.

In what follows, embodiments will be explained under the three followingcategories: introduction of the seed light 23 from between the rear sidemirror 111 and the chamber 3 (the rear part of the resonator),introduction of the seed light 23 from between the output side mirror 2and the chamber 3, and direct introduction of the seed light 23 in thechamber 3. The embodiments will be explained primarily with reference tothe structure of the amplification-stage laser 60, and with reference totop views unless otherwise stated. Discharge electrodes 4 and 5 (cathodeand anode), not shown, are located in the vertical direction to thepaper, and laser discharge occurs vertically to the paper. In theseembodiments, there is a higher degree of flexibility in the introductionof the seed light 23 in the direction vertical to the (cathode-to-anode)discharge direction than in that discharge direction, and so the seedlight 23 is introduced in the direction vertical to the dischargedirection. Notice here that the direction of introduction of the seedlight 23 is not necessarily limited to that vertical direction.

FIG. 48 is a top view illustrative of one embodiment wherein the seedlight 23 is injected from the side of the amplification-stage laser 60that is opposite to its laser exit side. The seed light 23 emitted outof the oscillation-stage laser 50 is injected in the amplification-stagelaser 60 via one or more total-reflection mirrors 121. In FIG. 48, theseed light 23 passes through the second total-reflection mirror 121 andtransmits through a window member 17 opposite to the laser exit side forinjection in the chamber 3. The injected seed light 23 passes the sideof the discharge area (gain area) 22 between the discharge electrodes 4and 5 (the underside of the paper) or through the discharge area 22 andthen through the window member 17 on the output side mirror 2 side,arriving at the output side mirror 2. Generally, the output side mirror2 is applied with a partial reflecting mirror coating 10 at one side andan antireflection coating 10 on the other or opposite side. Althoughwhether the partial reflecting mirror coating 10 of the output sidemirror 2 directs toward the chamber 3 side or in the laser outputdirection is not any essential requirement, that mirror coating 10 isapplied on the chamber 3 side in FIG. 48. Throughout the followingembodiments, the partial reflecting mirror coating 10 and theantireflection coating 9 of the output side mirror 2 are shown in FIG.48 alone.

The output side mirror 2 could be formed of an optical substrate withneither the partial reflecting mirror coating 10 nor the antireflectioncoating 9. With laser light of, for instance, 193 nm wavelength, thesurface reflection of the optical substrate is about 4%; if thesubstrate can make use of front- and back-surface reflection, it is thenpossible to achieve a 193 nm wavelength output mirror having areflectivity of about 8% without recourse to any coating.

The seed light 23 reflected at the partial reflecting mirror coating 10of the output side mirror 2 is directed toward the rear side mirror(total-reflection) mirror 111 positioned in the rear of the laserresonator. Then, the seed light 23 is subjected to multiple reflectionsbetween the output side mirror 2 and the rear side mirror 111 that formthe resonator, filling the discharge area 22.

As discharge occurs in the discharge area 22 in the amplification-stagelaser 60 during or after the discharge area 22 is filled with the seedlight 23, it allows the amplification-stage laser 60 to oscillatehigh-output, narrow-banded laser light having a line width inheritedfrom the seed light 23 from the oscillation-stage laser 50.

FIG. 49 is a top view illustrative of one embodiment wherein the seedlight 23 is injected in the amplification-stage laser 60 using surfacereflection at a window member 17 that opposes the laser exit side notlocated at the Brewster angle. The seed light 23 from the line narrowingoscillation-stage laser 50 is directed to the window member 17 on therear side of the resonator via one or more total-reflection mirrors 121.The directed seed light 23 is guided by surface reflection at the windowmember 17 to the rear side mirror 111. The seed light 23 is guided tothe output side mirror 2 upon reflection at the rear side mirror 111.Finally, the light 23 is subjected to multiple reflections between theoutput side mirror 2 and the rear side mirror 111 that form together theresonator.

Usually, the CaF₂ is used for the window member 17. In most cases, theseed light 23 is P-polarized light. FIG. 50 shows the reflectioncapability of CaF₂ to P-polarized light. Here, the angle of incidence ofthe seed light 23 on the window member 17 should preferably besubstantially equal to the angle of inclination with which the windowmember 17 is located (within ±5°). In other words, this injection modeworks for the window member that is not located with the Brewster angle.

FIG. 51(a) is a top view of an embodiment wherein the seed light 23 isinjected in the amplification-stage laser 60 while a high-reflectivity(total-reflection) coating 8 is applied to a part of the window member17 that opposes the laser exit side. When the window member 17 islocated in the chamber 3 with the Brewster angle or so, sufficientreflection of the seed light 23 will not be expected, as shown in FIG.50. In this case, a high-reflectivity (total-reflection) coating 8 isapplied to a site—capable of reflecting the seed light 23—of the part ofthe window member 17 that opposes the laser exit side, as shown in FIG.51(b). The rest area J is or is not be applied with an antireflectioncoating. Alternatively, only a site H through which theamplification-stage laser light passes is or is not applied with anantireflection coating, and other site is applied with ahigh-reflectivity (total-reflection) coating 8 in association with theinjection of the seed light 23.

In this embodiment, the seed light 23 is reflected at the portion of thehigh-reflectivity (total-reflection) coating 8 on the window member 17,and guided to the rear side mirror 111. Then, the seed light 23 isreflected at the rear side mirror 111 and guided to the output sidemirror 2. Finally, the light is subjected to multiple reflectionsbetween the output side mirror 2 and the rear side mirror 111.

FIG. 52(a) is a top view of an embodiment wherein the seed light 23 isinjected in the amplification-stage laser 60 from the rear portion ofthe resonator in the amplification-stage laser 60, in which resonatorthere is located a beam expander prism system (beam expander system) 61.In this embodiment, the beam expander prism systems 61 and 61 arelocated between one window member 17 and the rear side mirror 111 andbetween another window member 17 and the output side mirror 2,respectively, for the purpose of expanding the laser light incident onthe rear side mirror 111 and the laser light incident on the output sidemirror 2 in the amplification-stage laser 60. Each beam expander prismsystem 61 is here composed of two triangular prisms 62 and 63. A beamincident on one surface of the triangular prism 62 at right angles isincident from within on another surface at a relatively large angle ofincidence, which it leaves in a one-dimensional direction with anexpanded beam diameter. The beam with an expanded beam diameter isincident on one surface of another triangular prism 63 at right anglesand then incident from within on another surface with a relatively largeangle of incidence, which it leaves in a one-dimensional direction withan expanded beam diameter.

In this embodiment, the seed light 23 is directed by one or moretotal-reflection mirrors 121 to the beam expander prism system 61. Theprism 61 to which the seed light 23 is to be directed is applied with orwithout an antireflection coating on a transmitting area K of thesurface 64 on which laser light resonating in the resonator is to beincident, as shown in FIG. 52(b). The seed light 23 is incident on thattransmitting area K. The rest of the surface 64 of the prism 62 isapplied with a high-reflectivity (total-reflection) coating 8.Specifically but not exclusively in the embodiment of FIG. 52(b), thehigh-reflectivity (total-reflection) coating 8 is applied to the vertexside of the prism 62. Upon reflection at the high-reflectivity(total-reflection) coating 8 on the prism 62, the seed light 23 passesthrough the amplification-stage laser 60 and is guided to the outputside mirror 2. Finally, the light is subjected to multiple reflectionsbetween the output side mirror 2 and the rear side mirror 111 that formtogether the resonator.

Specifically but not exclusively in the embodiment of FIG. 52(a), theseed light 23 is guided from the prism 62 closer to the chamber 3 intothe chamber 3. When the beam expander prism system 61 is composed of twoor more prisms, the seed light 23 could be guided from any surface ofany prism into the amplification-stage laser 60.

FIG. 53 is a top view of an embodiment corresponding to FIG. 48. In thisembodiment, the seed light 23 is injected into the laser chamber 3 frombetween the output side mirror 2 and the laser chamber 3 in theamplification-stage laser 60. The seed light 23 is injected in theamplification-stage laser 60 via one or more total-reflection mirrors121. In FIG. 53, the seed light 23 passes through the secondtotal-reflection mirror 121 between the output side mirror 2 and thelaser chamber 3 and transmits through a window member 17 for injectionin the chamber 3. The injected seed light 23 passes the side of thedischarge area (gain area) 22 (the underside of the paper) or throughthe discharge area 22 and then through the window member 17 on the rearside mirror 111 side, arriving at the rear side mirror 111 that is atotal-reflection mirror located on the side opposing the output sidemirror with the resonator chamber 3 in the amplification-stage laser 60sandwiched between them. Then, the seed light 23 is reflected toward theoutput side mirror 2, and further reflected at a partial reflectingmirror coating 10 (FIG. 48) on the output side mirror 2. Thus, the seedlight 23 is subjected to multiple reflections between the output sidemirror 2 and the rear side mirror 11. Finally, the discharge area 22 isfilled with the seed light 23. Generally, the output side mirror 2 isapplied with the partial reflecting mirror coating 10 on one side and anantireflection coating 10 on the other or opposite side. Althoughwhether the partial reflecting mirror coating 10 on the output sidemirror 2 directs toward the chamber 3 side or in the laser outputdirection is not any essential requirement (See the explanation of FIG.48).

As discharge occurs in the discharge area 22 in the amplification-stagelaser 60 during or after the discharge area 22 is filled with the seedlight 23, it allows the amplification-stage laser 60 to oscillatehigh-output, narrow-banded laser light having a line width inheritedfrom the seed light 23 from the oscillation-stage laser 50.

FIG. 54 is a top view of an embodiment corresponding to FIG. 49. In thisembodiment, the seed light 23 from the oscillation-stage laser 50 isdirected to a window member 17 on the output side mirror 2 side via oneor more total-reflection mirrors 121. The directed seed light 23 isguided by surface reflection at the window member 17 to the output sidemirror 2. The seed light 23 is guided to the rear side mirror 111 uponreflection at the partial reflecting mirror coating 10 (FIG. 48) on theoutput side mirror 2. Thus, the seed light 23 is subjected to multiplereflections between the output side mirror 2 and the rear side mirror111 that form together the resonator.

In this case, too, the CaF₂ is usually used for the window member 17. Inmost cases, the seed light 23 is P-polarized light. FIG. 50 shows thereflection capability of CaF₂ to P-polarized light. Here, the angle ofincidence of the seed light 23 on the window member 17 should preferablybe substantially equal to the angle of inclination with which the windowmember 17 is located (within ±5°). In other words, this injection modeworks for the window member that is not located with the Brewster angle.

FIG. 55 is a top view of an embodiment corresponding to FIG. 51. In thisembodiment, the seed light 23 is injected in the amplification-stagelaser 60 while a high-reflectivity (total-reflection) coating 8 isapplied to a part of the window member 17 on the laser exit side. Whenthe window member 17 is located in the chamber 3 with the Brewster angleor so, sufficient reflection of the seed light 23 will not be expected,as shown in FIG. 50. In this case, the high-reflectivity(total-reflection) coating 8 is applied to a site—capable of reflectingthe seed light 23—of the part of the window member 17 on the laser exitside, as shown in FIG. 55(b). The rest area J is or is not be appliedwith an antireflection coating. Alternatively, only a site H throughwhich the amplification-stage laser light passes is or is not appliedwith an antireflection coating, and other site is applied with thehigh-reflectivity (total-reflection) coating 8 in association with theinjection of the seed light 23.

In this embodiment, the seed light 23 is reflected at the portion of thehigh-reflectivity (total-reflection) coating 8 on the window member 17,and guided to the output side mirror 2. Then, the seed light 23 isreflected at the output side mirror 2 and guided to the rear side mirror111. Thus, the light is subjected to multiple reflections between theoutput side mirror 2 and the rear side mirror 111.

FIG. 56 is a top view corresponding to FIG. 52. In this example,however, a beam expander prism system 61 is located between the windowmember 17 and the output side mirror 2 only for the purpose of expandingthe beam of laser light incident on the output side mirror 2 in theamplification-stage laser 60, and there is no such a bean expander prismsystem on the rear side mirror 111. The beam expander prism system 61 ishere composed of two triangular prisms 62 and 63. A beam incident on onesurface of the triangular prism 62 at right angles is incident fromwithin on another surface at a relatively large angle of incidence,which it leaves in a one-dimensional direction with an expanded beamdiameter. The beam with an expanded beam diameter is incident on onesurface of another triangular prism 63 at right angles and then incidentfrom within on another surface with a relatively large angle ofincidence, which it leaves in a one-dimensional direction with anexpanded beam diameter.

The prism 62 to which the seed light 23 from the oscillation-stage laser50 is to be directed has such configure as shown in FIG. 52(b). The seedlight 23 is reflected at the high-reflectivity (total-reflection)coating 8 on the prism 62 is guided to the rear side mirror 111 throughthe amplification-stage laser 60. Thus, the seed light 23 is subjectedto multiple reflections between the output side mirror 2 and the rearside mirror 111.

Specifically but not exclusively in the embodiment of FIG. 56, the seedlight 23 is guided from the prism 62 closer to the chamber 3 into thechamber 3. When the beam expander prism system 61 is composed of two ormore prisms, the seed light 23 could be guided from any surface of anyprism into the amplification-stage laser 60.

An embodiment of directing the seed light 23 directly in the chamber 3in the amplification-stage laser 60 is now explained.

FIG. 57 is a top view of an embodiment wherein the seed light 23 isinjected in the discharge area 22 through a seed light-injecting window65 attached to the side of the chamber 3 in the amplification-stagelaser 60. Anti-reflection coatings could be applied on both surfaces ofthe seed light-injecting window 65, although this is not alwaysnecessary. The seed light 23 is injected in the amplification-stagelaser 60 by one or more total-reflection mirrors 121. In FIG. 57, theseed light 23 is injected into the discharge area 22 in the chamber 3 inthe amplification-stage laser 60 via the second total-reflection mirror121 in the chamber 3. The injected seed light 23 passes through the sideof the discharge area 22 (the underside of the paper) or the dischargearea 22 and then transmits through the window member 17 on the rear sidemirror 111 side, arriving at the rear side mirror 111. The seed light 23reflected at the rear side mirror 111 goes toward the output side mirror2 located in front of the laser resonator. Thus, the seed light 23 issubjected to multiple reflections between the partial reflecting mirrorcoating 10 (FIG. 48) on the output side mirror 2 and the rear sidemirror 111, which form together resonator. Finally, the discharge area22 is filled with the seed light 23.

As discharge occurs in the discharge area 22 in the amplification-stagelaser 60 during or after the discharge area 22 is filled with the seedlight 23, it allows the amplification-stage laser 60 to oscillatehigh-output, narrow-banded laser light having a line width inheritedfrom the seed light 23 from the oscillation-stage laser 50.

Specifically but not exclusively in the embodiment of FIG. 57, the seedlight 23 is injected toward the rear side mirror 111. For instance, theseed light 23 could be injected toward the output side mirror 2.

FIG. 58 shows an embodiment wherein instead of the total-reflectionmirror 121, a total-reflection prism 122 is used as the member locatedin the chamber for total reflection of the seed light 23, and the restis the same as in FIG. 57. Therefore, only the total-reflection prism isnow explained. The total-reflection prism 122 is a CaF₂ prism having nocoating at all. As this total-reflection prism 122 is used as atotal-reflection optical element, it allows the service life of thatoptical element to be extended because of no deterioration due toproducts resulting from the high-reflectivity (total-reflection) mirrorcoating in the laser gas or laser system, which are found with thetotal-reflection mirror 121.

FIG. 59 is a top view of an embodiment wherein a partial reflecting film10 is coated to the input side mirror 1, so that the seed light 23transmits through the input side mirror 1 from its back surface for theinjection of the seed light 23. This will hereinafter be called the backsurface injection mode. The seed light 23 from the line narrowingoscillation-stage laser 50 is introduced and entered in the back surfaceof the input side mirror 1 that is a rear side mirror in the resonatorin the amplification-stage laser via one or more total-reflectionmirrors 121, while its optical axis is in substantial alignment with theoptical axis of the resonator in the amplification-stage laser. Thisinput side mirror 1 is applied with the partial reflecting film 10, sothat a part of the seed light 23 is injected into theamplification-stage laser resonator and the rest is reflected by thepartial reflecting film 10. Then, the seed light 23 is filled in betweenthe output side mirror 2 and the input side mirror 1 that form togetherthe resonator. As high voltage is applied between the electrodes 4 and5, it causes discharge, which allows the seed light 23 to be amplifiedby induction emission and the resonator to oscillate theamplification-stage laser 60.

Because the optical axis of the amplification-stage laser 60 is inalignment with the optical axis of the seed light 23, this mode providesthe following merits: (1) alignment is easily achievable, (2) thetolerance of misalignment of the optical axis of the seed light 23 iswide, and (3) there is a possibility of holding back the occurrence ofASE because 0.5 roundtrip is needed to fill the amplification-stagelaser resonator with the seed light 23. However, a problem with such aback surface injection mode is how the reflectivity of the input sidemirror 1 is optimized.

FIG. 60 is indicative of relations between the input side mirror 1 (thereflectivity of the rear mirror) and the post-synchronization laseroutput, with relative output normalized at the respective maximumoutputs as ordinate and the reflectivity of the mirror as abscissa. Thisis the post-synchronization output when the reflectivity of the inputside mirror 1 varies at an output side mirror's reflectivity of about30% while the output of the oscillation-stage laser is kept constant.From this graph, it has been found that the reflectivity of the mirrorproducing a maximum output is about 90%, and the reflectivity producing½ of the maximum output ranges from about 36% to about 98%. In otherwords; it has been found that the optimum value of the reflectivity ofthe input side mirror in the back surface injection mode is about 90%,and the usable reflectivity range for the input side mirror 1 is abouthalf the output having the optimum value, i.e., ranges from about 36% toabout 98%.

FIG. 61 is indicative of an effective enabling region for the angle andposition of injection of the seed light 23, θin and Xin, when the seedlight 23 is injected in the input side mirror 1 from its back surfacewith the same coordinate axes and under the same amplification-stagelaser conditions as in FIG. 35 (resonator length L=1,000 mm, dischargewidth Wx=2.5 mm, and the input side mirror 1 and the output side mirrorare arranged parallel with six roundtrips). The polygonal region in FIG.61 is larger than those found in FIGS. 37 and 38. This means that thetolerance of the optical axis of the seed light 23 to variations becomeswider in the back surface injection mode shown in FIG. 59 than in theoblique injection mode shown in FIG. 35. As a result, laser performance(such as energy stability and synchronous tolerance) will become better.

FIGS. 62 and 63 are each a top view of an embodiment of the mode whereinthe seed light 23 is introduced in the amplification-stage laser 60 viaa beam splitter 112 in the resonator in the amplification-stage laser60. In the embodiment of FIG. 62, the beam splitter 112 coated with apartial reflecting film 10 is interposed between the rear side mirror111 coated with a total-reflection film 8 and a rear side window 17 tointroduce the seed light 23 in the amplification-stage laser 60. In theembodiment of FIG. 63, the beam splitter 112 coated with the partialreflecting film 10 is interposed between a front side window 17 and theoutput side mirror 2 to introduce the seed light 23 in theamplification-stage laser 60. The seed light 23 from the line narrowingoscillation-stage laser 50 is guided and directed to the beam splitter112 located in the resonator in the amplification-stage laser 60 via oneor more total-reflection mirrors 121. The beam splitter 112 is providedwith the partial reflecting film 10 at which a part of the seed light 23is reflected, and the reflected light is then injected in theamplification-stage laser resonator while its optical axis is insubstantial alignment with the optical axis of the resonator. Theremaining transmitted seed light 23 is thrown away. Thus, the seed light23 is filled in between the rear side mirror 111 and the input sidemirror 2 that form together the resonator. As high voltage is appliedbetween the electrodes 4 and 5, it allows discharge to occur, so thatthe seed light 23 is amplified by induction emission and theamplification-stage laser 60 is oscillated by the resonator. In thisembodiment, losses due to the provision of the beam splitter 112 in theresonator in the amplification-stage laser 60 grow more than in theembodiment of FIG. 59, resulting in lower output. However, the aforesaidmerits (1), (2) and (3) are still kept intact.

In the embodiments of FIGS. 62 and 63, the seed light 23 is introducedin the resonator via the beam splitter 112. In a modification to them, apartial reflecting film is coated to the laser window 17 to allow it tohave a similar beam splitter role as mentioned above. In this case, too,the seed light 23 is injected in the amplification-stage laser resonatorwhile substantially coaxial with the resonator. Although the seed light23 is first introduced in the discharge area direction by means of thebeam splitter 112, it is acceptable to introduce the seed light 23 inthe direction of the rear side mirror 111 or the output side mirror 2.In any case, the light reflected from the mirror toward theamplification-stage laser is amplified. It is then required, however, tomake the output of the oscillation-stage laser 50 higher, because lossesof the seed light 23 grow more than in the embodiments of FIGS. 62 and63.

FIG. 64 is a top view of an embodiment wherein the seed light 23 ispermitted to transmit through the output side mirror 1 by the beamsplitter 112 for injection in the amplification-stage laser resonator.The seed light 23 from the line narrowing oscillation-stage laser 50 isguided and directed to the output side mirror 2 in the resonator in theamplification-stage laser 60 via one or more total-reflection mirrors121 while its optical axis is in substantial alignment with the opticalaxis of the amplification-stage resonator. The beam splitter 112 isprovided with a partial reflecting film 10, and the portion of the seedlight 23 transmitting through the beam splitter 112 is thrown away whilethe reflected light is entered in the output side mirror 2. The portionof the seed light 23 transmitting through the output side mirror 1 isinjected in the amplification-stage resonator. The remaining portion ofthe seed light 23 is reflected by the output side mirror 1. Thus, theseed light 23 is filled in between the output side mirror 2 and the rearside mirror 111 that form together the resonator. As high voltage isapplied between the electrodes 4 and 5, it allows discharge to occur, sothat the seed light 23 is amplified by induction emission and theamplification-stage laser 60 is oscillated by that resonator. In thisembodiment, losses of the seed light 23 grow more than in the embodimentof FIG. 59, because of poor efficiency of injection through the beamsplitter 112 and the output side mirror 2. It is thus required to makethe output of the oscillation-stage laser 50 higher; however, theaforesaid merits (1), (2) and (3) are still kept intact.

By the way, when the diameter of laser light from the oscillation-stagelaser 50 is equal to the diameter of output laser light from theamplification-stage laser 60 so that the conversion optical system 70could be dispensed with, the front mirror 52 in the oscillation-stagelaser 50 and the input side mirror 1 in the amplification-stage laser 60could be provided by a common or sharing mirror. FIG. 65 is a top viewillustrative in schematic of this embodiment, wherein a cascadeconnection is made between the oscillation-stage laser 50 and theamplification-stage laser 60 in such a way as to share the common mirror52-1. The front surface of the transparent substrate of the commonmirror 52-1 is applied with a partial reflecting mirror coating to forma partial reflecting mirror surface for the front mirror 52 in theoscillation-stage laser 50, and the back surface of the transparentsubstrate of the common mirror 52-1 is provided with a high-reflectivitymirror coating (except the seed light-introduction hole 7″) as showntypically in FIG. 5, for application to the input side mirror 1 in theamplification-stage laser 60. It is here noted that the front surface ofthe common mirror 52-1 could have a surface configuration for the frontmirror 52 in the resonator for the oscillation-stage laser 50, and theback surface could be configured in a planar or concave shape for theinput side mirror 1 in the amplification-stage laser 60.

FIG. 66 is illustrative of a modification to the back surface injectionmode. When the diameter of laser light from the oscillation-stage laser50 is substantially equal to that from the amplification-stage laser 60so that the conversion optical system 70 could be dispensed with, thefront mirror 52 in the oscillation-stage laser 50 and theamplification-stage laser 60 could be designed to have a common orsharing input side mirror 52-2 coated with a partial reflecting film 10.It is noted that the partial reflecting film 10 could be applied to theside of the input side mirror that opposes the side shown. In thisembodiment, a cascade connection is made between the oscillation-stagelaser 50 and the amplification-stage laser 60 in such a way as to sharethe common mirror 25-2 coated with the partial reflecting film 10 on itsone surface. The line narrowing module 51 and the partial reflectingsurface of the common mirror 25-2 work as a resonator to oscillate theoscillation-stage laser 50 to produce the seed light from the partialreflecting surface of the common mirror 52-2. At the same time, the seedlight is entered directly in the resonator in the oscillation-stagelaser 60, which is made up of the common mirror 52-2 and the output sidemirror 2. As high voltage is applied between the electrodes 4 and 5, itpermits discharge to occur, so that the seed light is amplified byinduction emission and the amplification-stage laser 60 is oscillated bythat resonator. In this case, the reflectivity of the common mirrorcould be effective if it comes in the range of FIG. 60.

In addition to the merits (1), (2) and (3) of the back surface injectionmode, this mode provides additional merits as set forth just below.Because the common mirror 52-2 having the partial reflecting film isshared by the front mirror in the oscillation-stage laser 50 and theinput side mirror in the amplification-stage laser 60, (1) any means forthe introduction of seed light can be dispensed with, making the systemcompact and less costly, (2) the seed light can be injected in theresonator in the amplification-stage laser without losses, so that theoscillation-stage laser can be kept low in output and compact in size,and (3) the optical axes of the oscillation- and amplification-stagelasers are substantially in alignment, so that they can be easilyadjusted with higher stability.

A difference in the advantage between the embodiments of FIGS. 65 and 66is now explained. The output of the oscillation-stage laser can be lowerin the embodiment of FIG. 66 than in that of FIG. 65. This is becausethe common mirror 52-2 having the partial reflecting film on its onesurface is shared by the front mirror in the oscillation-stage laser 50and the input side mirror in the amplification-stage laser 60, so thatall the seed light 23 can be injected in the resonator in theamplification-stage laser 60 without causing losses of the output of theoscillation-stage laser 50. Thus, the oscillation-stage laser 50 can bemade smaller and less costly.

With the above two-stage laser system for aligners according to theinvention, Fabry-Perot etalon type stable resonator or a resonator withits two mirrors slightly inclined with each other is used in theamplification-stage laser so as to achieve a spatial coherence as low asthat of the oscillation-stage laser, and light having divergence is usedas the seed light oscillated from the oscillation-stage laser so as tofill a laser gas gain area with the seed light for efficientamplification. Even with a ring resonator using a plurality of planemirrors in the amplification-stage laser, too, the desired low spatialcoherence is achievable.

FIG. 67 is a side view illustrative in schematic of one embodiment usingsuch a ring resonator. In this embodiment, the seed light from theoscillation-stage laser 50 is directed through a reflecting mirror 99 tothe conversion optical system 70 where it is reduced to the desired beamwidth, entering the amplification-stage laser 60. Theamplification-stage laser 60 comprises a ring resonator built up of aninput/output partial reflecting mirror 91, a total-reflection mirror 92for the reflection of seed light transmitting through the partialreflecting mirror 91, and a total-reflection right-angle prism (roofprism) 93 having two total-reflection surfaces and operable to reflectincident light in a direction substantially parallel with and oppositeto the direction of incidence, wherein all reflecting surfaces areformed of planes. Thus, the gain area (discharge area) in the chamber 3positioned between the partial reflecting mirror 91/total-reflectionmirror 92 and the total-reflection right-angle prism 93 can be filledwith the seed light having divergence while it makes roundtrips in thering resonator, which the amplified laser light leaves as output via thepartial reflecting mirror 91.

FIG. 68 is a plan view illustrative in schematic of another embodimentusing the ring resonator. The seed light from the oscillation-stagelaser 50 is entered in the amplification-stage laser 60 via a reflectingmirror 99. The amplification-stage laser 60 comprises a ring resonatorbuilt up of an input/output partial reflecting mirror 91 and threetotal-reflection mirrors 92, 93 and 94 for sequentially reflecting theseed light transmitting through the partial reflecting mirror 91 back tothe partial reflecting mirror 91, wherein all reflecting surfaces areformed of planes. Thus, the gain area (discharge area) in the chamber 3positioned between the partial reflecting mirror 91/total-reflectionmirror 92 and the total-reflection mirrors 94, 95 can be filled with theseed light having divergence while it makes roundtrips in the ringresonator, which the amplified laser light leaves as output via thepartial reflecting mirror 91.

The inventors have further found that if, in a two-stage laser systemhaving its spatial coherence decreased while taking advantage of thehigh stability, high output efficiency and fine line width of the MOPOsystem explained in the preamble of the disclosure, the length of anoptical path in the resonator in the amplification-stage laser isspecified as described below, it is then possible to provide a two-stagelaser system more suitable for use on semiconductor aligners.

As a result of experiments after experiments, the inventors havediscovered that there is often an interference fringe pattern in thebeam profile configuration of laser light produced out of theamplification-stage laser, although depending on the length of theoptical path in the resonator in the amplification-stage laser.

This interference fringe pattern, if any, renders the symmetry of thebeam profile configuration worse. Further, the interference fringepattern moves with time due to changes in the center wavelength of theseed light 23 produced out of the oscillation-stage laser 50, changes inthe resonator length of the amplification-stage laser 60 or the like,rendering the stability of the beam profile worse too.

The beam profile configuration of the laser light produced out of thetwo-stage laser system that is a light source for the aligner has someconsiderable influences on the uniform illumination of masks on thealigner and, hence, on exposure capability on what is to be exposed(wafers). Further, fluctuations of the interference fringe pattern giverise to too large fluctuations of laser light output to control.

Why the interference fringe pattern occurs is now explained withreference to FIGS. 69 and 70. FIG. 69 is illustrative in schematic of aMOPO type two-stage laser system to which the invention is applied, andlaser light characteristics as well. Specifically, FIG. 69(a) is aschematic illustration of the MOPO type two-stage laser system to whichthe invention is applied; FIG. 69(b) is indicative of a spectral profileof narrow-banded laser light produced out of the oscillation-stagelaser; and FIG. 69(c) is indicative in section of laser light (beamprofile configuration) produced out of the amplification-stage laser.

For instance, given the narrow-banded laser light (seed light 23)produced out of the oscillation-stage laser 50 has the spectral profileshown in FIG. 69(b). The seed light 23 is injected in the resonator inthe amplification-stage laser 60 (which is built up of, e.g., the inputside mirror 1 and the output side mirror 2), wherein it is amplified andoscillated.

Here, when the resonator in the amplification-stage laser 60 is built upof a stable resonator or a resonator with its mirrors slightly inclinedwith respect to each other and that resonator is comprised of an inputside (total-reflection) mirror 1 and an output side (partial reflecting)mirror 2, the seed light 23 transmits through the input side mirror 1and thereafter passes through the discharge area 22 in theamplification-stage laser 60, where it is amplified. The amplified lightafter passing through the discharge area 22 is incident on the outputside mirror 2 that is a partial reflecting mirror, and a part of thereflected light is produced as the first laser light L1 through theoutput side mirror 2.

On the other hand, the amplified light reflected by the output sidemirror 2 passes through the discharge area 22 where it is amplified,entering the input side mirror 1. The amplified light subjected to totalreflection at the input side mirror 1 passes through the discharge area22 wherein it is amplified, entering the output side mirror 1. A part ofthat light transmits through the output side mirror 2, leaving it as thesecond laser light K2. The remaining amplified light is reflected by theoutput side mirror 2 toward the amplification area 22. In the resonatorin the amplification-stage laser 60, such resonance occurs repeatedly.

The first laser light K1 and the second laser light K2 interfere whenthe optical path difference between the both laser light K1 and K2 isshorter than a time-based coherent length Lc corresponding to thespectral width of the seed light 23 produced out of theoscillation-stage laser 50.

Here, let λ be the wavelength of the laser light, and Δλ be the spectralline width. Then, the time-based coherent length Lc is defined byequation (9) (see non-patent publication 1).Lc=λ ²/Δλ  (9)

As in the evaluation of spatial coherence, interference fringecapability on the B-B section of FIG. 69(a) is evaluated in terms ofvisibility and optical path difference. The optical path differenceherein is tantamount to the distance that the laser light (seed light)travels from entering the resonator to leaving it; that is, it issubstantially twice as long as the resonator length L of theamplification-stage laser 60. Visibility is also found from thefollowing formula:Visibility=(maximum fringe intensity I _(max) of interference fringepattern−minimum fringe intensity I _(min) of interference fringepattern)÷(maximum fringe intensity I _(max) of interference fringepattern+minimum fringe intensity of I _(min) of interference fringepattern

FIG. 70 is indicative of relations between the length twice as long asthe resonator length L of the amplification-stage laser and thevisibility of the interference fringe pattern. FIG. 70 teaches that asthe length twice as long as the resonator length L of theamplification-stage laser 60 (that length substantially matches theoptical path difference between the first laser light K1 and the secondlaser light K2) becomes long, the visibility of the interference fringepattern occurring on the beam profile of laser light produced out of theamplification-stage laser 60 becomes small. It also teaches that as thelength substantially twice as long as the resonator length L of theamplification-stage laser 60 becomes longer than the time-basedcoherence length Lc of the seed light 23 produced out of theoscillation-stage laser 50, the interference fringe pattern virtuallydisappears.

Referring typically to an ArF laser MOPO type two-stage laser system foraligners, when the spectral line width (full width half maximum) of theseed light 23 produced out of the oscillation-stage laser 50 is Δλ=0.2pm and the wavelength is λ=193.4 nm, the time-based coherence length Lcis found from equation (9) to be Lc=about 0.186 m. Therefore, to preventany interference fringe pattern from occurring in the beam profile ofthe laser light output, the resonator length L of theamplification-stage laser 60 must be 0.186/2=0.093 m or longer.

When the ring resonator is used as the resonator in such anamplification-stage laser 60 as shown in FIGS. 67 and 68, suchconditions as mentioned below must be satisfied to prevent anyinterference fringe pattern from occurring on the bema profile of thelaser light produced out of the amplification-stage laser 60.

With the ring resonator, the interference fringe pattern can be heldback by making its optical path length longer than the time-basedcoherent length Lc corresponding to the spectral line width of thenarrow-banded seed light 23 produced out of the oscillation-stage laser50.

With the embodiment of FIG. 67, the length of the optical path taken bythe seed light 23 (laser light) from the position where it leaves thepartial reflecting mirror 91 upon entrance and transmission through ituntil that laser light again arrives at the partial reflecting mirror 9via the total-reflection mirror 92 and the total-reflection right-angleprism 93 should preferably be longer than the time-based coherent lengthLc.

With the embodiment of FIG. 68, the length of the optical path taken bythe seed light 23 (laser light) from the position where it leaves thepartial reflecting mirror 91 upon entrance and transmission through ituntil that laser light again arrives at the partial reflecting mirror 9via the total-reflection mirrors 92, 94 and 95 should preferably belonger than the time-based-coherent length Lc.

That is, if, in FIGS. 67 and 68, the optical path length difference uponthe laser light split by the partial reflecting mirror 91 crossing againover it is made longer than the time-based coherent length Lccorresponding to the spectral line width of the seed light 23 producedout of the oscillation-stage laser, it is then possible to prevent anyinterference fringe pattern from occurring on the beam profile of thelaser light produced out of the amplification-stage laser 60.

While the two-stage laser system for aligners according to the inventionhas been described with referent to its principles and embodiments, itis to be understood that the invention is by no means limited to themand various modifications to them are possible.

For instance, when the two-stage laser system for aligners according tothe invention is a fluorine molecule (F₂) laser system, theoscillation-stage laser 50 could comprise, in place of the linenarrowing module 51, a line select module comprising at least one angledispersion element and a total-reflection mirror located in order fromits side on which laser light is incident.

Specifically, the laser light produced out of the F₂ laser system hastwo primary oscillation wavelengths (λ₁=157.6299 nm and λ₂=157.5233 nm:non-patent publication 2). The spectral line width (FWHN) of both linesis about 1 pm. When an alignment optical system in the aligner iscatadioptric system, chromatic aberrations are prevented even at suchspectral line widths as mentioned above.

In this case, therefore, the oscillation line of stronger intensity λ₁(=157.6299 nm) among both lines is usually selected by the aforesaidline select module upon free-run oscillation.

It is noted that such a line select module is not necessarily located inthe oscillation-stage laser 50; it could be located in the outputoptical path of the output side mirror 60 in the amplification-stagelaser 60.

Here, when any interference fringe pattern is prevented from occurringon the beam profile of the laser light produced out of theamplification-stage laser 60, the resonator length Lc of theamplification-stage laser 60 is determined as mentioned above, whilecomparing with the time-based coherent length Lc of theoscillation-stage laser 50. Upon determination of the time-basedcoherent length Lc from equation (9), the spectral lie width Δλ of theoscillation-stage laser 60 is determined as follows.

Here consider the case where the two-stage laser system for aligners isa F₂ laser system comprising a line select module on the rear side ofthe oscillation-stage laser 50. From a comparison of the output of laserlight (seed light 23) at a wavelength λ₁ (=157.6299 nm) selected by theline select module with the output of laser light (seed light 23) at awavelength λ₂ (=157.5233 nm) selected by the line select module, it isfound that the output of laser light having a wavelength λ₂ is merelyabout 20% lower than that of laser light a wavelength λ₁. In this case,therefore, it is possible to select the wavelength λ₂ by the line selectmodule. In other words, the above spectral line width Δλ is that of theoscillation line at the wavelength λ₁ (=157.6299 rum) or λ₂ (=157.5233nm) selected by the line select module.

When the two-stage laser system is a F2 laser system comprising a lineselect module externally of the output side mirror 2 in theamplification-stage laser 60, the spectral line width Δλ is that of theoscillation line at wavelength λ₁ of stronger intensity among twoprimary oscillation lines of wavelength λ₁=157.6299 nm and wavelengthλ₂=157.5233 nm.

POSSIBLE INDUSTRIAL APPLICATIONS

In the two-stage laser system for aligners according to the invention,oscillation laser light having divergence is used as theoscillation-stage laser and the amplification-stage laser comprises aFabry-Perot etalon resonator where the resonator is configured as astable resonator or, alternatively, oscillation laser light havingdivergence is used as the oscillation-stage laser and theamplification-stage laser comprises a ring resonator comprising aninput/output partial reflecting mirror and a plurality oftotal-reflection mirrors for reflecting laser light entered via thepartial reflecting mirror back to the position of the partial reflectingmirror wherein the partial reflecting mirror and the plurality oftotal-reflection mirrors are each formed of a plane. Thus, the two-stagelaser system for aligners according to the invention has the features ofthe MOPO mode that output fluctuations are insensitive to fluctuationsof synchronous excitation timing between the chambers, high energystability and high output efficiency are achievable, laser (seed) energyfrom the oscillation stage can be kept lower, the spectral line width isnarrow because the latter half of a laser pulse from theoscillation-stage laser makes a lot more roundtrips, and the line widthis narrow because the tail of the latter half can be amplified, and hasthe features of the MOPA mode as well that the spatial coherence is low;that is, given the same share quantity (pinhole-to-pinhole space) in thebeam transverse direction, the visibility of interference fringes andthe spatial coherence are low.

If the optical axis of laser light oscillated out of theoscillation-stage laser and entered in the amplification-stage laser isset in such a way as to make an angle with the optical axis of theresonator in the amplification-stage laser, then the spatial coherenceis much more reduced.

If the length about twice as long as the length of the resonator in theamplification-stage laser is set longer than the time-based coherentlength corresponding to the spectral line width of the oscillation-stagelaser or the length of the optical path through the ring resonator isset longer than the time-based coherent length corresponding to thespectral line width of the oscillation-stage laser, it is then possibleto prevent any interference fringe pattern from occurring on the beamprofile of laser light produced out of the amplification-stage laser. Itis thus possible to maintain the symmetry of the beam profile and holdback its fluctuations and, hence, provide uniform illumination of masksin an aligner. Thus, the invention provides a two-stage laser systemwell fit especially for semiconductor aligners.

The invention is in no sense limited to the use of the oscillation laserlight having divergence as the oscillation-stage laser. For instance, ifthe optical axis of laser light oscillated out of the oscillation-stagelaser and entered in the amplification-stage laser is set in such a wayas to make an angle with respect to the optical axis of the resonator inthe amplification-stage laser, it is then possible to obtain a two-stagelaser system that does not only have the above features of the MOPO modebut also is reduced in terms of spatial coherence so that it lendsitself well to semiconductor aligners.

Further, if the reflecting surfaces of the rear side mirror and theoutput side mirror are each formed of a plane, the normal lines to therear side mirror and the output side mirror are set in such a way as tomake an angle with respect to the optical axis of laser light oscillatedout of the oscillation-stage laser and entered in theamplification-stage laser and with each other as well, and the laserlight oscillated out of the oscillation-stage laser is entered in theresonator from the side on which the distance between both mirrors islonger, it is then possible to obtain a two-stage laser system that doesnot only have the above features obtained by setting the optical axis oflaser light entered in the amplification-stage laser in such a way as tomake an angle with respect to the optical axis of the resonator in theamplification-stage laser but also has an increased laser output and anextended pulse width and ensures the degree of flexibility in theinjection of laser light entered in the amplification-stage laser with adecrease in the peak intensity of the oscillation-stage laser, and so isbest suited for use with semiconductor aligners.

1-45. (canceled)
 46. A two-stage laser system for aligners, comprisingan oscillation-stage laser and an amplification-stage laser that inputslaser light oscillated by said oscillation-stage laser, amplifies andoutputs the laser light, and said oscillation-stage laser and saidamplification-stage laser each comprising a discharge electrode in achamber filled with a laser gas, wherein the output light of saidoscillation-stage laser has a divergence angle, said amplification-stagelaser comprises a ring resonator including a partial reflecting mirrorthat serves both as an input of the oscillation-stage laser light and anoutput of laser light amplified, a first total-reflection mirror thatreflects the laser light input via said partial reflecting mirror andtransmits said laser light through a discharge area of the dischargeelectrode of said amplification-stage laser, and a plurality oftotal-reflection mirrors that transmits again the light transmittedthrough said discharge area by the first total-reflection mirror of saidamplification-stage laser to the discharge area of the dischargeelectrode and return the light back to the position of said partialreflecting mirror, and said partial reflecting mirror and said pluralityof total-reflection mirrors are each formed of a plane.
 47. Thetwo-stage laser system for aligners according to claim 46, wherein theoptical path of the ring resonator of said amplification-stage laserlies in a plane perpendicular to the discharge direction of thedischarge electrode of said amplification-stage laser.
 48. The two-stagelaser system for aligners according to claim 46 or 47, wherein saidfirst total-reflection mirror is a total-reflection mirror that reflectsthe laser light input via said partial reflecting mirror and transmitssaid laser light inclined to the longitudinal direction of the dischargeelectrode of said amplification-stage laser through said discharge area,and said plurality of total-reflection mirrors are a plurality oftotal-reflection mirrors that transmits again the light transmittedthrough said discharge area by the first total-reflection mirror of saidamplification-stage laser to the discharge area along the longitudinaldirection of said discharge electrode and return the light back to theposition of said partial reflecting mirror.
 49. The two-stage lasersystem for aligners according to one of claims 46 to 48, wherein saidoscillation-stage laser is a narrow-band laser.
 50. The two-stage lasersystem for aligners according to one of claims 46 to 49, wherein betweensaid oscillation-stage laser and said amplification-stage laser, thereis located a conversion optical system having a function of compressinga beam shape of laser light oscillated out of said oscillation-stagearea.
 51. The two-stage laser system for aligners according to one ofclaims 46 to 50, wherein an optical path length in said ring resonatoris set longer than a time-based coherent length corresponding to aspectrum line width of said oscillation-stage laser.